N.-P. Liu

BI VULPECULAE: A SIAMESE TWIN WITH TWO VERY SIMILAR COOL STARS IN SHALLOW CONTACT

BI Vul is a cool eclipsing binary star (Sp. = K3 V) with a short period of 0.2518 days. The first charge-coupled device (CCD) light curves of the binary in the BVRI obtained on 2012 August 21 are presented and are analyzed using the Wilson-Devinney code. It is discovered that BI Vul is a marginal contact binary system (f = 8.7%) that contains two very similar cool components (q = 1.037). Both the marginal contact configuration and the extremely high mass ratio suggest that it is presently evolving into contact with little mass transfer between the components and it is at the beginning stage of contact evolution. By using all available times of minimum light, the variations in the orbital period are investigated for the first time. We find that the observed - calculated (O - C) curve of BI Vul shows a cyclic change with a period of 10.8 yr and an amplitude of 0.0057 days, while it undergoes a downward parabolic variation. The cyclic oscillation is analyzed for the light-travel time effect that arises from the gravitational influence of a possible third stellar object. The mass and orbital separation of the third body are estimated asM(3) similar to 0.30 M-circle dot and similar to 4.9 AU, respectively. The downward parabolic change reveals a long-term period decrease at a rate of (P) over dot = -9.5 x 10(-8) days yr(-1). The period decrease may be caused by angular momentum loss via magnetic stellar wind and/or it is only a part of a long-period (longer than 32 yr) cyclic variation, which may reveal the presence of another stellar companion in a wider orbit. These observational properties indicate that the formation of the Siamese twin is driven by magnetic braking and the third stellar companion should play an important role by removing angular momentum from the central binary.

OPTICAL FLARES AND A LONG-LIVED DARK SPOT ON A COOL SHALLOW CONTACT BINARY

W UMa-type stars are contact systems where both cool components fill the critical Roche lobes and share a common convective envelope. Long and unbroken time-series photometry is expected to play an important role in their origin and activity. The newly discovered short-period W UMa-type star, CSTAR 038663, was monitored continuously by Chinese Small Telescope ARray (CSTAR) in Antarctica during the winters of 2008 and 2010. There were 15 optical flares recorded in the i band during the winter of 2010. This was the first time such flares were detected from a W UMa-type star. By analyzing the nearly unbroken photometric data from 2008, it is discovered that CSTAR 038663 is a W-type shallow contact binary system (f = 10.6(+/- 2.9)%) with a high mass ratio of q = 1.12(+/- 0.01), where the less massive component is slightly hotter than the more massive one. The asymmetric light curves are explained by the presence of a dark spot on the more massive component. Its temperature is about 800 K lower than the stellar photosphere and it covers 2.1% of the total photospheric surface. The lifetime of the dark spot is longer than 116 days. Using 725 eclipse times, we found that the observed-calculated (O-C) curve may show a cyclic variation that is explained by the presence of a close-in third body. Both the shallow contact configuration and the extremely high mass ratio suggest that CSTAR 038663 is presently evolving into a contact system with little mass transfer. The formation and evolution is driven by the loss of angular momentum via magnetic braking, and the close-in companion star is expected to play an important role, removing angular momentum from the central eclipsing binary.

FIRST PHOTOMETRIC INVESTIGATION OF THE NEWLY DISCOVERED W UMa-TYPE BINARY STAR MR Com

By analyzing multi-color light curves of the newly discovered W UMa-type binary, MR Com, we discovered that it is a shallow-contact binary with a degree of contact factor of f = 10.0% +/- 2.1%. Photometric solutions reveal that MR Com is a W-type system with a mass ratio of q = 3.9 where the less massive component is about 90 K hotter than the more massive one. By investigating all of the available times of minimum light, we found that the general trend of the Observed-Calculated (O - C) curve shows a downward parabolic variation while it undergoes a cyclic variation with a small amplitude of 0.0031 days and a period of 10.1 yr. The downward parabolic change corresponds to a long-term decrease in the orbital period at a rate of (P) over dot = -5.3 x 10(-7) days yr(-1) that may be caused by a combination of a mass transfer and an angular momentum loss (AML) via magnetic braking. Among the 16 shallow-contact systems with a decreasing orbital period, MR Com has the lowest mass ratio (e.g., 1/q = 0.26). The shallow-contact configuration, the low-mass ratio, and the long-term period decrease all suggest that systems similar to MR Com are on the AML-controlled stage of the evolutionary scheme proposed by Qian. They will oscillate around a critical mass ratio and evolve into a deep contact with a higher mass ratio. The small-amplitude cyclic change in the O - C curve was analyzed for the light-travel time effect via the presence of an extremely cool stellar companion.

Photometric Study of the Possible Cool Quadruple System PY Virginis

Complete CCD photometric light curves in BV(RI)(c) bands obtained in 2012 for the short-period close binary system PY Virginis are presented. A new photometric analysis with the Wilson-Van Hamme code shows that PY Vir is an A-type marginal contact binary system. The absolute parameters of PY Vir are derived using spectroscopic and photometric solutions. Combining new determined times of minimum light with others published in the literature, the O - C diagram of the binary star is investigated. A periodic variation, with a period of 5.22(+/- 0.05) years and an amplitude of 0.0075(+/- 0.0004) days, was discovered. Since the spectrum of a third component has been detected by Rucinski et al., we consider this cyclic period oscillation to be the result of the light-time effect due to the presence of a third body. This third component may also be a binary itself. Therefore, PY Vir should be a quadruple system composed of two cool-type binary systems. This system is a good astrophysical laboratory to study the formation and evolution of close binaries and multiple systems.

THE LATE K-TYPE BINARY V1104 HER NEAR THE SHORT-PERIOD END OF CONTACT BINARIES

V1104 Her is a newly discovered periodic variable with EW type variability. With an orbital period that is extremely short for a contact binary, it was found to have almost the latest K spectral type. To study its properties and nature, two new sets of multi-color light curves were analyzed with the Wilson-Devinney code. Photometric solutions reveal that the system is a W-type shallow contact binary with a mass ratio of 1/q=M-1/M-2=0.63. +/- 0.01 and a degree of contact of about f=15% +/- 2%. The real parameter uncertainties may be from three to five times larger, depending on the required confidence. Combining the newly determined times of minimum light, some of which were reprocessed from initial data, in conjunction with others published, the period variation is investigated in detail. Different methods of curve fitting were used, based on a weighted least-squares method. Finally, the orbital period of V1104 Her is found to be undergoing a long-term decrease at a rate of dP/dt = -2.9(+/- 0.5) x 10(-8) days . yr(-1), as well as double cyclic oscillations with a period ratio (T-1/T-2) very close to 9:2. The light-travel time effect was found to be the best explanation for the cyclic variations. This indicates that the object is a possible quadruple system, which further implies that multiplicity may be a common phenomenon among close binaries.

FIRST PHOTOMETRIC STUDY OF THE VERY SHORT PERIOD K-TYPE CONTACT BINARY 1SWASP J064501.21+342154.9

The first high precision CCD photometric light curves of 1SWASP J064501.21+342154.9 were obtained in 2013 February along with several new times of minimum light. The multi-color light curves were analyzed using the Wilson-Devinney code. Photometric solutions reveal that the system is a W-type shallow contact binary with a mass ratio of q = 2.110 ± 0.007 and a degree of contact of f = 15.3% ± 2.1%. A third light contributing about 2% of the total luminosity in the I band was found. It may come from a cool tertiary component. A small spot found on the surface of the binary indicates that the solar-like activities are at a low level at present.

THREE CLOSE BINARIES IN DIFFERENT EVOLUTIONARY STAGES IN THE OLD OPEN CLUSTER NGC 188

NGC 188 is a good laboratory for studying the formation and evolution of W UMa type contact binaries due to its rich populations of them. We present a detailed photometric study of three short-period close binaries, EP Cep, ES Cep, and V369 Cep, in the old open cluster NGC 188 based on our two-set photometric observations. We discovered that both EP Cep and ES Cep are shallow-contact binaries with continuously decreasing periods. The difference is in their mass ratios. EP Cep has an extremely low-mass ratio, q = 0.15, while ES Cep has a relatively high-mass ratio, q = 0.69, indicating that they lie in different evolutionary stages. ES Cep is likely a newly formed contact binary via a Case A mass transfer, while EP Cep is an evolved system and may be on the oscillations caused by the combined effect of the thermal relaxation oscillation and the variable angular momentum loss. For another system, V369 Cep, we found that it is a primary-filling near-contact binary. Both the semidetached configuration and the continuous decrease in the orbital period indicate that it is undergoing a mass transfer from the primary component to the secondary one. This conclusion is in agreement with the excess luminosity seen in the light curves on the ingress of the secondary minimum produced by the impact of the mass transfer. All of the results suggest that V369 Cep is evolving into contact, and a shallow-contact high-mass ratio system similar to ES Cep will be formed. Then, it will evolve into a low-mass ratio contact binary just like EP Cep, and finally merge into a rapidly rotating single star.

A CCD PHOTOMETRIC STUDY OF THE CONTACT BINARY STAR GSC 03526-01995

We present the first two sets of complete light curves in the R and I bands observed in 2011 and 2012 photometrically for the contact binary GSC 03526-01995. The asymmetric light curves are analyzed with spot models using the Wilson-Devinney code. It is discovered that GSC 03526-01995 is a spotted marginal W-type W UMa contact binary system with a mass ratio of q = 2.845(+/- 0.006) and a degree of contact factor f = 18.2%(+/- 1.5%). Combining new determined times of light minimum together with the others published in the literature, the period change of the system is investigated. A cyclic variation, with a period of 7.39 yr and a semiamplitude of 0.00896 days, is discovered. It is plausibly caused by the light-time effect via a third body. The photometric analysis indicates that the suspected third companion might itself be a binary consisting of two stars of 0.285 solar masses, suggesting that GSC 03526-01995 is a quadruple system containing four late-type stars.