J.-Z. Yuan

AD Cancri: A shallow contact solar-type eclipsing binary and evidence for a dwarf third component and a 16 year magnetic cycle

CCD photometric observations of AD Cancri obtained from 2000 March 7 to 2004 December 20 are presented. Variations of the light levels at the primary minimum and both maxima are found. Uniform solutions of four sets of photometric data were derived by using the Wilson-Devinney method. The solutions suggest that AD Cancri is a shallow W-type contact binary (f 8.3% +/- 1.3%) with a highmass ratio of 1/q = 0.770 +/- 0.002. The long-term variation of the light curve is explained by variable dark-spot models of the more massive component star with a possible 17 yr cycle. Our 13 times of light minimum over 5 years, including others collected from the literature, have been used for the period study. The complex period changes can be sorted into a long-term period increase at rate of dP/dt +(4.94 +/- 0.16) x 10(-7) days yr(-1), a 16.2 yr periodic component (A(3) = 0.0155 days), and a very small amplitude period oscillation (A(4) = 0.0051 days, P-4 = 6.6 yr). The existence of third light may indicate that there is a tertiary component in the binary system. Solving the four-band light curves of Samec & Bookmyer, it is found that the contribution of the tertiary component to the total light of the triple system increases with wavelength, which suggests that it is very cool and may be a very red main-sequence star. The small-amplitude period oscillation may be caused by the light-time effect of the cool tertiary component (M-3 similar to 0.41 M-circle dot). The 16.2 yr periodic component in the orbital period and the 17 yr cyclic activity of the dark spot on the more massive component both may reveal that the more massive component displays solar-type magnetic activity with a cycle length of about 16 yr.

A New CCD Photometric Investigation of the Short-Period Close Binary AP Leonis

New CCD photometric light curves in the B, V, and R bands of the short-period close binary AP Leonis are presented. A photometric analysis with our symmetric light curves suggests that AP Leo is an overcontact binary with a degree of overcontact of 24.9%. Since the O - C values of photographic and visual times of light minimum showed a large scatter (up to 0.06 days), all of the period changes proposed for the eclipsing binary by previous investigators are not reliable. In this paper the orbital period changes of AP Leo are analyzed based on all published CCD and photoelectric eclipse times. A small-amplitude cyclic oscillation, with a period of 22.4 yr and an amplitude of 0.0049 days, is discovered to be superposed on a secular period decrease at a rate of dP/dt = - 1:08; 10(-7) days yr(-1). The continuous period decrease may be caused by angular momentum loss or a combination of the mass transfer from the primary to the secondary and angular momentum loss. The cyclic period change may indicate that AP Leo is a triple system containing a cool dwarf third component. If this is true, it is possible that this third component plays an important role in the origin and evolution of the overcontact system by removing angular momentum from the central system, and that it makes the eclipsing pair have a low angular momentum and a short initial orbital period ( e. g., P < 5 days). In that case, the initially detached system evolves into an overcontact configuration via magnetic torques from stellar winds. On the other hand, the rapid rotation of the solar-type components (spectral type G0) and the variations of the light curve indicate a high degree of magnetic activity from the spin-up of the components. Both the long-term period decrease and the oscillation can plausibly be interpreted by magnetic activity (i.e., enhanced magnetic stellar wind and activity cycles).

Deep, low mass ratio overcontact binary systems. v. the lowest mass ratio binary V857 herculis

Charge-coupled device (CCD) photometric light curves in the B, V, and R bands of the complete eclipsing binary star V857 Her are presented. It is shown that the light curves of the W UMa-type binary are symmetric and of A type according to Binnendijks classification. Our four epochs of light minimum along with others compiled from the literature were used to revise the period and study the period change. Weak evidence indicates that the orbital period of V857 Her may show a continuous increase at a rate of dP/dt = +2.90 x 10(-7) days yr(-1). The photometric parameters of the system were determined with the 2003 version of the Wilson-Devinney code. It is shown that V857 Her is a deep overcontact binary system with f = 83.8% +/- 5.1%. The derived mass ratio of q = 0.06532 +/- 0.0002 suggests that it has the lowest mass ratio among overcontact binary systems. As the orbital period increases, the decrease of the mass ratio will cause it to evolve into a single rapidly rotating star when it meets the more familiar criterion that the orbital angular momentum be less than 3 times the total spin angular momentum. To understand the evolutionary state of the system, long-term photometric monitoring and spectroscopic observations will be required.

Ternarity, Activity, and Evolutionary State of the W UMa-Type Binary UX Eridani

Charge-coupled device photometric observations of the W UMa-type binary star UX Eridani are presented. Comparing the B light curve with that obtained by Binnendijk in 1964-1965, the variation of the light curve around the primary minimum was found. Photometric solutions of Binnendijks and our light curves were derived by using the new version of the Wilson-Devinney program. Our solutions confirmed that UX Eri is a marginal W-type overcontact binary system with a very low degree of overcontact, f 44.5 circle dot. The tertiary component may have played an important role in the formation of the progenitor of UX Eri by transferring angular momentum during the Kozai oscillation. In that way, the detached progenitor could evolve into overcontact configuration via magnetic braking. It was found that the timescale of the period increase is close to the thermal timescale of the less massive component, which suggests that UX Eri is in an evolutionary state of thermally conservative mass transfer from the less massive component to the more massive one.

CCD Photometric Study of the Contact Binary TX Cnc in the Young Open Cluster NGC 2632

TX Cnc is a member of the young open cluster NGC 2632. In the present paper, four CCD epochs of light minimum and a complete V light curve of TX Cnc are presented. A period investigation based on all available photoelectric or CCD data showed that it is superimposed on a long-term increase (dP/dt = +3.97 x 10(-8) d yr(-1)), and weak evidence suggests that it includes a small-amplitude period oscillation (A(3) = 0.(d)0028; T-3 = 26.6 yr). The light curves in the V band obtained in 2004 were analyzed with the 2003 version of the W-D code. It was shown that TX Cue is an overcontact binary system with a degree of contact factor f = 24.8% (+/- 0.9%). The absolute parameters of the system were calculated: M-1 = 1.319 +/- 0.007 M-circle dot M-2 = 0.600 +/- 0.01 M-circle dot; R-1 = 1.28 +/- 0.19 R-circle dot, R-2 = 0.91 +/- 0.13 R-circle dot. TX Cnc may be on the TRO-controlled stage of the evolutionary scheme proposed by Qian (2001a, b, 2003a), and may contain an invisible tertiary component (m(3) approximate to 0.097 (circle dot)). If this is true, the tertiary component has played an important role in the formation and evolution of TX Cnc by removing angular momentum from the central system (Pribulla & Rucinski 2006). In this way the contact binary configuration can be formed in the shortlife time of a young open cluster via AML.

CCD Photometric Study of the W UMa-Type Binary II Canis Majoris in the Field of Berkeley 33

The CCD photometric data of the EW-type binary, II CMa, which is a contact star in the field of the middle-aged open cluster Berkeley 33, are presented. The complete R light curve was obtained. In the present paper, using the five CCD epochs of light minimum ( three of them are calculated from Mazur et al. (1993)’s data and two from our new data), the orbital period P was revised to 0.22919704 days. The complete R light curve was analyzed by using the 2003 version of W-D (Wilson-Devinney) program. It is found that this is a contact system with a mass ratio q = 0.9 and a contact factor f = 4.1%. The high mass ratio (q = 0.9) and the low contact factor (f = 4.1%) indicate that the system just evolved into the marginal contact stage. Subject headings: Stars: binaries : close – Stars: binaries : eclipsing – Stars: individual (II CMa) – Stars: evolution

A Photometric Study of the Near Contact Binary UU Lyncis

The near-contact binary UU Lyn with an F3V-type primary was observed in 2005 and 2006. With the latest version of the Wilson-Devinney code, the photometric elements were computed. The results reveal that UU Lyn is a marginal contact system with a large temperature difference of about 1900 K between the primary and secondary components. All available eclipse times, including new ones, were used in the analysis. The results show that the orbital period of this system undergoes a continuous decrease at a rate of dP/dt = -1.84 x 10(-8) d yr(-1). With the period decrease, UU Lyn may evolve from the present short-period marginal contact system into a contact system with true thermal contact. This target might just be undergoing the cycles predicted by the theory of thermal relaxation oscillations (TRO). It is an interesting example resembling BL And, GW Tau, ZZ Aur, KQ Gem, CN And and AD Cnc, that lie in the key evolutionary stage.