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

Abstract

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?