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1

Search for Dormant Black Holes in the OGLE Data

Kapusta M., Mróz P.

Acta Astronomica

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2023

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Vol. 73, No. 3

197--226

EN

Light curves of ellipsoidal variables collected by the Optical Gravitational Lensing Experiment (OGLE) were analyzed, in order to search for dormant black hole candidates. After the preselection based on the amplitude of ellipsoidal modulation, each object was investigated by means of the spectral energy distribution fit, which allowed us to select objects that are in close agreement with the spectrum of a single stellar object. After this final step of the preselection process, we were left with only fourteen objects that were then investigated in detail. For each candidate, we estimated basic physical parameters such as temperature, mass, luminosity, and, in some cases, radial velocity semi-amplitude. One of the objects turned out to be a spotted star while the rest are considered black-hole binary candidates. In the end, we present an alternative explanation for the ellipsoidal modulation in the form of contact binaries, which are not only vast in number, contrary to black-hole binaries, but are also in much better agreement with the radial velocity estimates for some of the systems analyzed. Even if the presented arguments suggest a noncompact character of the companion objects, each of them should be observed spectroscopically in order to verify the compact object hypothesis.

2

On the Orbital Period and Models of V Sge

Smak J.

Acta Astronomica

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2022

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Vol. 72, No. 1

21--29

EN

The orbital period of V Sge is decreasing at a rate which increased from dP/dt=-(4.11±0.33)×10-10 in 1962 to -(5.44±0.61)× 10-10 in 2022. This implies that the mass transfer from the secondary component is accelerating. From the evidence based on the orbital period variations, combined with estimates of the mass loss from the system based on radio observations, it follows that (1) the mass transfer rate from the secondary component is larger than M2=-5×10-6 M☉/yr, possibly as large as M2=-2.5×10-5 M☉/yr, and (2) the mass loss rate from the primary component is M1=-4×10-7 M☉/yr or larger. Close similarity of V Sge to binary Wolf-Rayet stars supports the model with primary component being a hot, evolved star loosing its mass. Several arguments are presented which exclude the alternative model with primary component being a white dwarf with an accretion disk.

3

Light Curve Modeling of Four Short-Period W UMa Binaries

Čeki A., Latković O.

Acta Astronomica

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2022

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Vol. 72, No. 3

183--194

EN

We use extensive model grids to estimate the global parameters of four partially-eclipsing W UMa contact binaries near the period cutoff. All four systems consist of K-type main sequence primaries and M-type secondaries that appear undersized and underluminous for their masses because of the energy transfer through the common envelope. Three of the four stars exhibit light curve asymmetry that is explained in terms of magnetic activity and modeled with dark spots. We discuss the reliability of the photometric mass ratios and derived absolute parameters in context of total or partial eclipses and compare them with a sample of totally-eclipsing short-period W UMa systems from the literature.

4

On the Pulsations of the δ Scuti Star of the Binary System KIC 6629588

Liakos A.

Acta Astronomica

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2020

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Vol. 70, No. 4

265--289

EN

his work includes a comprehensive analysis of the Kepler detached eclipsing binary system KIC 6629588 that aims at the detailed study of the oscillation properties of its pulsating component. Ground-based spectroscopic observations were obtained and used to classify the components of the system. The spectroscopic results were used as constraints for the modeling of the short-cadence Kepler light curves and for the estimation of the absolute parameters of the components. Furthermore, the light curve residuals are analyzed using Fourier transformation techniques in order to search for pulsation frequencies. The primary component of the system is identified as a δ Sct star that pulsates in seven eigenfrequencies in the range of 13-22 d-1, while more than 270 combination frequencies were also detected. The absolute and the oscillation parameters of this pulsating star are compared with those of other δ Sct stars members of detached binary systems using evolutionary and correlation diagrams. Finally, the distance to the system is also estimated.

5

DV Psc: A Magnetically Active Hierarchical Triple System

Gazeas K., Palafouta S.

Acta Astronomica

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2019

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Vol. 69, No. 3

261--282

EN

The eclipsing binary system DV Psc has been known to be magnetically active for almost two decades. However, there has been no evidence of a magnetic cycle on this system until recently. This study focuses on the long-term photometric monitoring of DV Psc between 2005-2017. A total of 50 individual light curves in BVRI optical bands were collected, in order to investigate its magnetic activity and cycle, as well as determine its orbital and physical properties. The combined photometric and spectroscopic observations of this study resulted in a unified model for the system, which describes accurately the light curves throughout the years, as a result of the variable spot activity. A total of 105 new times of minimum light are calculated through the entire time span of observations and they are combined with the 203 bibliographic ones since 1997, increasing significantly the existing sample. This resulted in an accurate ephemeris and an updated O-C diagram for a total span of 20 yr (1997-2017). It is found that the system exhibits intense magnetic activity, which is shown through the strong asymmetries on the light curves (O'Connell effect) and the periodic variation of the O-C diagram. The existence of a third body, orbiting the eclipsing binary with a period of Porb=9.79±0.60 yr in an eccentric orbit with eccentricity e=0.83±0.24, as well as a magnetic cycle of 14.74±0.84 yr are most likely connected with this variability. The absolute physical parameters of the system are calculated for a new and unified model, which explains the light curves through the entire observing season. The evolution state of DV Psc is studied through the mass-radius and temperature-luminosity (HR) diagrams, as well as the location of the two components with respect to the ZAMS and TAMS region.

6

The Cluster AgeS Experiment (CASE). Detecting Aperiodic Photometric Variability with the Friends of Friends Algorithm

Rozyczka M., Narloch W., Pietrukowicz P., Thompson I. B., Pych W., Poleski R.

Acta Astronomica

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2018

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Vol. 68, No. 1

63--78

EN

We adapt the friends of friends algorithm to the analysis of light curves, and show that it can be succesfully applied to searches for transient phenomena in large photometric databases. As a test case we search OGLE-III light curves for known dwarf novae. A single combination of control parameters allows us to narrow the search to 1% of the data while reaching a ≈90% detection efficiency. A search involving ≈2% of the data and three combinations of control parameters can be significantly more effective - in our case a 100% efficiency is reached. The method can also quite efficiently detect semi-regular variability. In particular, 28 new semi-regular variables have been found in the field of the globular cluster M22, which was examined earlier with the help of periodicity-searching algorithms.

7

Roche Modeling and Evolutionary History of Multiple Low Mass Contact Binary Systems

Alton K. B., Stępień K.

Acta Astronomica

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2018

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Vol. 68, No. 4

449--482

EN

The Northern Sky Variable Survey (NSVS) has been a rich source of variable stars many of which have only been studied to a limited extent. Herein we describe five eclipsing binary systems that were first detected by the ROTSE-I telescope and ultimately shown to be low mass contact binaries. Photometric data (B, V and Ic) acquired from MU Cnc (NSVS 10133793, TYC 1397-1030-1), V596 Peg (NSVS 6225359, GSC 2765-0348), TYC 1597-2327-1 (NSVS 11082150), GSC 2723-2376 (NSVS 8849526) and GSC 4946-0765 (NSVS 13176410) were used to calculate an orbital period for each system along with new linear ephemerides. Except for MU Cnc, the limited number of time-of-minimum values precluded the characterization of any long-term period change. Since a total eclipse is observed with all five systems, Roche modeling based on the Wilson-Devinney W-D code yielded in each instance a well-constrained photometric value for the mass ratio. Using the evolutionary model of cool close binaries we searched for possible progenitors of the investigated binaries. The best fit is obtained if the initial binaries have orbital periods from the interval between 2.4–3.4 d, total masses between 6% and 9% higher than presently observed and mass ratios between 2.2 and 3.3. The model progenitors need between 7.5–10 Gyr to attain the presently observed parameters of the variables. The present periods of three models slowly increase with simultaneous mass ratio decrease, which suggests the formation of the common envelope in the future. However, in four other models the period decreases and the model computations predict the ultimate overflow of the outer Roche lobe.

8

GT Ursae Majoris AB - a Possible Quadruple System

Dimitrov W., Tomov T., Kamiński K., Polińska M., Iliev I., Kamińska M. K.

Acta Astronomica

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2018

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Vol. 68, No. 2

141--158

EN

We present the first spectroscopic study of an eclipsing system GT UMa. Our spectra show that the main visual component A is a triple-lined star, which consists of a close eclipsing pair and a farther third component. Radial velocity measurements show evidence of reflex motion. We used Hipparcos and our own R-band light curves for the analysis. These results enabled us to calculate absolute parameters of the eclipsing binary system: 1.37 M⊙ and 1.31 M⊙ and corresponding radii 2.11 R⊙ and 1.63 R⊙. Both eclipsing components are evolved. The orbital period of the eclipsing subsystem is 1.2 d and the semi-major axis - 6.5 R⊙. The wide orbit has low radial velocity amplitude and its parameters are difficult to determine. We obtained a preliminary solution which must be confirmed with future observations. The preliminary period of the wide orbit is 394 d and the lower limit for the semi-major axis - 174 R⊙ (0.81 a.u.). We estimated the mass of the third body to about 1.1 M⊙. Additionally, we investigated the visual companion B. We obtained spectra of this star and measured the radial velocity which is very close to the one of the component A. The proper motions and the parallax are also similar. These are strong arguments that component B is gravitationally bound with the main triple system A.

9

CCD Photometry, Light Curve Modeling and Period Analysis of the Overcontact Eclipsing Binary BN Ari

Alton K. B., Nelson R. H., Boyd D. R. S.

Acta Astronomica

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2018

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Vol. 68, No. 2

159--181

EN

We present photometric and spectroscopic data of BN Ari, a totally eclipsing variable star. 15 new times-of-minimum have been determined. These along with other published eclipse timings were used to update the linear ephemeris and evaluate changes in orbital periodicity. Radial velocity data along with a definitive classification spectrum are reported for the first time. Simultaneous modeling of multicolor light curves and radial velocity data was accomplished using the Wilson-Devinney code with optimization by differential corrections. The weight of evidence from evaluating both the eclipse timing differences and light curve modeling indicates that BN Ari is most likely a triple system.

10

Superhumps and their Evolution during Superoutbursts

Smak J.

Acta Astronomica

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2016

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Vol. 66, No. 1

75--83

EN

Light curves of superhumps and their evolution during superoutbursts are analyzed by decomposing them into their Fourier components, including the fundamental mode and the first three overtones. The amplitudes of the fundamental mode are found to decrease significantly during superoutburst while those of the overtones remain practically constant. The phases of maxima of the fundamental mode increase systematically during superoutburst while those of the overtones - systematically decrease. The combination of the two effects is responsible for the characteristic evolution of superhump light curves: the appearance and growth of the secondary humps and the spurious phase jumps in the (O-C) diagrams. Two intrepretations are possible. Either that instead of just one superhump period Psh there are four periods Pk which resemble - but are significantly different from - the fundamental mode and the first three overtones of Psh. Or - more likely - that those time-dependent phase shifts are genuine.

11

P13 an ULX that is a Potential Progenitor of Merging BH-NS System

Beldycki B., Belczynski K.

Acta Astronomica

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2016

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Vol. 66, No. 3

347--355

EN

We have studied the future evolution of a recently discovered ULX source P13 in NGC 7793. This source was shown to contain a 5-15 M⊙ black hole and a massive 18-23 M⊙ B9Ia companion on a 64 day orbit. For low black hole mass (5-10 M⊙) and high companion mass ≳20 M⊙ the binary is predicted to initiate a common envelope evolution in near future and significantly decrease the orbital separation (6 hr orbit). This leads to a high probability (≈70%) of the system surviving a supernova explosion that will form a neutron star out of the companion. About one third of the surviving BH-NS systems will merge within Hubble time and be a source of high frequency gravitational radiation. We estimate that the chances of detection of BH-NS systems with Advanced LIGO/Virgo that form via P13-evolutionary channel are at the level of 0.1 yr-1 with wide range of allowed probability 0÷0.6 yr-1. This is the fourth empirical estimate of BH-NS merger rate.

12

Hot Spot in Eclipsing Dwarf Nova IY Ursae Majoris during Quiescence and Normal Outburst

Bąkowska K., Olech A.

Acta Astronomica

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2015

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Vol. 65, No. 4

351--360

EN

We present the analysis of hot spot brightness in light curves of the eclipsing dwarf nova IY UMa during its normal outburst in March 2013 and in quiescence in April 2012 and in October 2015. Examination of four reconstructed light curves of the hot spot eclipses showed directly that the brightness of the hot spot changed significantly only during the outburst. The brightness of the hot spot, before and after the outburst, was on the same level. Thus, based on the behavior of the hot spot, IY UMa during its normal outburst follows the disk-instability model.

13

The Clusters AgeS Experiment (CASE). VI. Analysis of Two Detached Eclipsing Binaries in the Globular Cluster M55

Kaluzny J., Thompson I. B., Dotter A., Rozyczka M., Pych W., Rucinski S. M., Burley G. S.

Acta Astronomica

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2014

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Vol. 64, No. 1

11--26

EN

We present an analysis of the detached eclipsing binaries V44 and V54 belonging to the globular cluster M55. For V54 we obtain the following absolute parameters: Mp=0.726±0.015 Msun, Rp=1.006± 0.009 Rsun, Lp=1.38±0.07 Lsun for the primary, and Ms=0.555± 0.008 Msun, Rs=0.528±0.005 Rsun, Ls=0.16±0.01 Lsun for the secondary. The age and apparent distance modulus of V54 are estimated at 13.3-14.7 Gyr and 13.94±0.05 mag, respectively. This derived age is substantially larger than ages we have derived from the analysis of binary systems in 47 Tuc and M4. The secondary of V44 is so weak in the optical domain that only mass function and relative parameters are obtained for the components of this system. However, there is a good chance that the velocity curve of the secondary could be derived from near-IR spectra. As the primary of V44 is more evolved than that of V54, such data would impose much tighter limits on the age and distance of M55.

14

The Clusters AgeS Experiment (CASE). Analysis of the Detached Eclipsing Binary V15 in the Metal-Rich Open Cluster NGC 6253

Rozyczka M., Kaluzny J., Thompson I. B., Dotter A., Pych W., Narloch W.

Acta Astronomica

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2014

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Vol. 64, No. 3

233--246

EN

We present the first detailed analysis of the detached eclipsing binary V15 in the super-metal rich open cluster NGC 6253. We obtain the following absolute parameters: Mp=1.303±0.006 MSun, Rp=1.71±0.03 RSun, Lp=2.98±0.10 LSun for the primary, and Ms=1.225±0.006 MSun, Rs=1.44±0.02 RSun, Ls=2.13±0.06 LSun for the secondary. Based on Dartmouth isochrones, the age of NGC 6253 is estimated to be 3.80-4.25 Gyr from the mass-radius diagram and 3.9-4.6 Gyr from color-magnitude diagram fitting. Both of these estimates are significantly higher than those reported so far. The derived apparent distance modulus of 11.65 mag agrees well with the range of 10.9-12.2 mag derived by other authors; however our estimated reddening (0.113 mag) is lower than the lowest published value (0.15 mag). We confirm earlier observations that model atmospheres are not accurate enough to account for the whole CMD of the cluster, with the largest discrepancies appearing on the subgiant and giant branches. Although age estimation from the mass-radius diagram is a relatively safe, distance- and reddening-independent procedure, our results should be verified by photometric and spectroscopic observations of additional detached eclipsing binaries which we have discovered, at least two of which are proper-motion members of NGC 6253.

15

Hot Spot Manifestation in Eclipsing Dwarf Nova HT Cassiopeiae

Bąkowska K., Olech A.

Acta Astronomica

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2014

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Vol. 64, No. 3

247--259

EN

We report the detection of a hot spot in the light curves of the eclipsing dwarf nova HT Cas during its superoutburst in 2010 November. Analysis of the eight reconstructed light curves of the hot spot eclipses showed directly that the brightness of the hot spot was changing significantly during the superoutburst. Thereby, detected hot spot manifestation in HT Cas is the newest observational evidence for the EMT model for dwarf novae.

16

CzeV404 - an Eclipsing Dwarf Nova in the Period Gap During its July 2014 Superoutburst

Bąkowska K., Olech A., Pospieszyński R., Martinelli F., Marciniak A.

Acta Astronomica

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2014

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Vol. 64, No. 4

337--348

EN

Results of the CCD observations of CzeV404 are presented. During the season of June-August 2014 we detected one outburst and one superoutburst of the star. Clear superhumps with the period of Psh=0.10472(2) days were observed. The superhump period was decreasing with a high value of dP/dt=-2.43(8)×10-4. From 17 eclipses we calculated the orbital period with the value of Porb=0.0980203(6) days which confirms that CzeV404 belongs to period gap objects and it is the longest orbital period of an eclipsing SU UMa star. Based on superhump and orbital period determinations, the period excess ε=6.8%±0.02% and the mass ratio q≈0.32; of the system were obtained.

17

Evolution of Cool Close Binaries - Rapid Mass Transfer and Near Contact Binaries

Stępień K., Kiraga M.

Acta Astronomica

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2013

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Vol. 63, No. 2

239--266

EN

We test the evolutionary model of cool close binaries developed by one of us (KS) on the observed properties of near contact binaries (NCBs). These are binaries with one component filling the inner critical Roche lobe and the other almost filling it. Those with a more massive component filling the Roche lobe are SD1 binaries whereas in SD2 binaries the Roche lobe filling component is less massive. Our evolutionary model assumes that, following the Roche lobe overflow by the more massive component (donor), mass transfer occurs until mass ratio reversal. A binary in an initial phase of mass transfer, before mass equalization, is identified with SD1 binary. We show that the transferred mass forms an equatorial bulge around the less massive component (accretor). Its presence slows down the mass transfer rate to the value determined by the thermal time scale of the accretor, once the bulge sticks out above the Roche lobe. It means, that in a binary with a (typical) mass ratio of 0.5 the SD1 phase lasts at least 10 times longer than resulting from the standard evolutionary computations neglecting this effect. This is why we observe so many SD1 binaries. Our explanation is in contradiction to predictions identifying the SD1 phase with a broken contact phase of the Thermal Relaxation Oscillations model. The continued mass transfer, past mass equalization, results in mass ratio reversed. SD2 binaries are identified with this phase. Our model predicts that the time scales of SD1 and SD2 phases are comparable to one another. Analysis of the observations of 22 SD1 binaries, 27 SD2 binaries and 110 contact binaries (CBs) shows that relative number of both types of NCBs favors similar time scales of both phases of mass transfer. Total masses, orbital angular momenta and orbital periods of SD1 and SD2 binaries are indistinguishable from each other whereas they differ substantially from the corresponding parameters of CBs. We conclude that the results of the analysis fully support the model presented in this paper.

18

Dwarf Novae in the OGLE Data. II. Forty New Dwarf Novae in the OGLE-III Galactic Disk Fields

Mróz P., Pietrukowicz P., Poleski R., Udalski A., Soszyński I., Szymański M. K., Kubiak M., Pietrzyński G., Wyrzykowski Ł., Ulaczyk K., Kozłowski S., Skowron J.

Acta Astronomica

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2013

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Vol. 63, No. 2

135--158

EN

We report the discovery of forty erupting cataclysmic variable stars in the OGLE-III Galactic disk fields: seventeen objects of U Gem type, four of Z Cam type, and nineteen stars showing outbursts and superoutbursts typical for SU UMa type dwarf novae. In the case of five stars we were able to estimate their supercycle lengths. The obtained lengths are in the range of 20-90 d, generally between the typical SU UMa type variables and a few objects classified as the ER UMa type variables. Since there is no significant difference between the two types but a higher mass-transfer rate resulting in more frequent outbursts and superoutbursts in the ER UMa type stars, we propose to discard this type as a separate class of variables. We note that in one of the SU UMa type stars, OGLE-GD-DN-039, we found a negative supercycle period change, in contrast to other active systems of this type. Two of the new OGLE objects showed long-duration WZ Sge-like superoutbursts followed by a sequence of echo outbursts. All stars reported in this paper are part of the OGLE-III Catalog of Variable Stars.

19

Evolution of Low Mass Contact Binaries

Stępień K., Gazeas K.

Acta Astronomica

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2012

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Vol. 62, No. 2

153--177

EN

We present a study on low-mass contact binaries (LMCB) with orbital periods shorter than 0.3 days and total mass lower than about 1.4 MSun. We show that such systems have a long pre-contact phase, which lasts for 8-9 Gyrs, while the contact phase takes only about 0.8 Gyr, which is rather a short fraction of the total life. With low mass transfer rate during contact, moderate mass ratios prevail in LMCBs since they do not have enough time to reach extreme mass ratios often observed in higher mass binaries. During the whole evolution both components of LMCBs remain within the MS band. The evolution of cool contact binaries toward merging is controlled by the interplay between the evolutionary expansion of the less massive component resulting in the mass transfer to the more massive component and the angular momentum loss from the system by the magnetized wind. In LMCB the angular momentum loss prevails. As a result, the orbital period systematically decreases until the binary overflows the outer critical Roche surface and the components merge into a single fast rotating star of a solar type surrounded by a remnant disk carrying excess angular momentum. The disk can be a place of planet formation with the age substantially lower than the age of a host star. The calculated theoretical tracks show good agreement with the physical properties of LMCB from the available observations. Estimates of the frequency of occurrence of LMCB and the merger formation rate indicate that about 40 LMCBs and about 100 low mass merger products is expected to exist within 100 pc from the Sun.

20

The Optical Gravitational Lensing Experiment. Dwarf Novae in the OGLE Data. I. Three New Dwarf Novae: One in the Period Gap and Two Longer Period Objects

Poleski R., Udalski A., Skowron J., Szymański M. K., Kubiak M., Pietrzyński G., Soszyński I., Kozłowski S., Pietrukowicz P., Wyrzykowski Ł., Ulaczyk K.

Acta Astronomica

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2011

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Vol. 61, No. 2

123--138

EN

We report serendipitous discovery of three new dwarf novae which eruptions in 2010 were observed by the ongoing microlensing survey OGLE-IV. All three objects are located in the Galactic bulge fields observed with the highest cadence of 20 minutes. In the OGLE-III and OGLE-IV data we revealed a total of 23 outbursts for one of the stars. What makes this object most interesting is the derived superhump period of 2.61 h placing it in the orbital period gap. The superhump period changed during the superoutburst with a very short timescale. For two other objects, for which we observed outburst, the orbital periods of 5.4 h and 9.5 h were measured in the quiescence.