W.-P. Liao

Detection of a planetary system orbiting the eclipsing polar HU Aqr

Using the precise times of mid-egress of the eclipsing polar HU Aqr, we discovered that this polar is orbited by two or more giant planets. The two planets detected so far have masses of at least 5.9 and 4.5M(Jup). Their respective distances from the polar are 3.6 and 5.4 au with periods of 6.54 and 11.96 yr, respectively. The observed rate of decrease of period derived from the downward parabolic change in the observed - calculated (O - C) curve is a factor of 15 larger than the value expected for gravitational radiation. This indicates that it may be only a part of a long-period cyclic variation, revealing the presence of one more planet. It is interesting to note that the two detected circumbinary planets follow the Titus-Bode law of solar planets with n = 5 and 6. We estimate that another 10 yr of observations will reveal the presence of the predicted third planet.

The most plausible explanation of the cyclic period changes in close binaries: the case of the RS CVn-type binary WW Dra

Cyclic period changes are a fairly common phenomenon in close binary systems and are usually explained as being caused either by the magnetic activity of one or both components or by the light travel time effect (LTTE) of a third body. We searched the orbital period changes in 182 EA-type (including the 101 Algol systems used by Hall), 43 EB-type and 53 EW-type binaries with known mass ratio and spectral type of the secondary component. We reproduced and improved the diagram in Hall according to the new collected data. Our plots do not support the conclusion derived by Hall that cyclic period changes are restricted to binaries having a secondary component with spectral type later than F5. The presence of period changes among systems with a secondary component of early type indicates that magnetic activity is one, but not the only, cause of the period variation. It is discovered that cyclic period changes, probably resulting from the presence of a third body, are more frequent in EW-type binaries among close systems. Therefore, the most plausible explanation of the cyclic period changes is the LTTE through the presence of a third body. Using the century-long historical record of the times of light minimum, we analysed the cyclic period change in the Algol binary WW Dra. It is found that the orbital period of the binary shows a similar to 112.2-yr cyclic variation with an amplitude of similar to 0.1977 d. The cyclic oscillation can be attributed to the LTTE by means of a third body with a mass no less than 6.43M(circle dot). However, no spectral lines of the third body were discovered, indicating that it may be a candidate black hole. The third body is orbiting the binary at a distance closer than 14.4 au and may play an important role in the evolution of this system.

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.

MAGNETIC BRAKING AND THE EVOLUTION OF THE HW VIR-LIKE BINARY STARS

HW Vir-like close binaries are thought to be the progenitors of cataclysmic variables. However, how this type of peculiar stars evolve into cataclysmic variables is an unsolved problem in astrophysics. Here we report the variations of the orbital period of HW Vir, the prototype of this group of systems. A long-term period decrease is discovered to be superimposed on the well-known cyclic change reported by previous investigators. It is suggested that the continuous decrease can be plausibly interpreted by angular momentum loss via magnetic braking of the completely convective star. This is driving the shrink of the orbit and will result in mass transfer between both components. We found that the binary will be undergoing mass transfer long before the sdB primary star evolves into a white dwarf. This conclusion suggests that HW Vir-like binaries do not directly evolve into cataclysmic variables through angular momentum loss as astronomers previously thought. On the other hand, if the observed period decrease is only part of a long-period cyclic variation, it may reveal the presence of another substellar companion in a wide orbit. This makes HW Vir an important system for future study of the formation and evolution of substellar objects.

A NEW PHOTOMETRIC INVESTIGATION OF THE W UMA-TYPE BINARY BI CVn

New photometric observations and their investigation of the W UMa-type binary, BI CVn, are presented. The variations of the orbital period were analyzed based on 12 new determined times of light minimum together with the others compiled from the literature. It is discovered that the period of BI CVn shows a long-term period decrease at a rate of. (P) over dot = -1.51(+/- 0.12) x 10(-7) days year(-1) while it undergoes a cyclic variation with a period of 27.0 years and an amplitude of 0(d).0151. Photometric solutions determined with the Wilson-Devinney method suggest that BI CVn is a contact binary with a degree of contact of 18.0(+/- 1.7)%. The asymmetry of the light curves was interpreted by the presence of dark spots on both components, and absolute parameters were determined by combining the photometric elements with the spectroscopic solutions given by Lu. The observed period decrease can be plausibly explained by a combination of the mass transfer from the primary to the secondary and angular momentum loss via magnetic braking. The cyclic period oscillation suggests that BI CVn is a triple system containing a tertiary component with a mass no less than 0.58 M(circle dot) in a 27.0 year orbit. As in the cases of the other contact binaries (e. g., AH Cnc, AP Leo, AD Cnc, and UX Eri), it is possible that this tertiary companion played an important role for the formation and evolution of the contact system by removing angular momentum from the central system via Kozai oscillation or a combination of Kozai cycle and tidal friction, which causes the eclipsing pair to have a short initial orbital period (e. g., P < 5(d)). In that case, can the initially detached system evolve into the present contact configuration via a combination of magnetic torques from stellar winds and a case A mass transfer?

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.

PERIOD CHANGES AND FOUR-COLOR LIGHT CURVES OF THE ACTIVE OVERCONTACT BINARY V396 MONOCEROTIS

This paper analyzes the first secured four color light curves of V396 Mon using the 2003 version of the WD code. It is confirmed that V396 Mon is a shallow W-type contact binary system with a mass ratio

Evidence of a Massive Black Hole Companion in the Massive Eclipsing Binary V Puppis

q=2.554(\pm0.004)

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

and a degree of contact factor

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

f=18.9%(\pm1.2%)