VW Bootis: The Shortest Period Close Binary System at the Beginning of the Overcontact Phase

Abstract

A new orbital period of the very short-period (0.342 day) close binary system VW Bootis is determined and its O-C curve is formed based on the analysis of all available times of light minimum. It is discovered that the orbital period of the system shows a secular decrease with a rate of dP/dt = 1.56 x 10(-7) days per year. Since the order of the period decrease is larger than the thermal conservative mass-transfer estimate, the mass transfer would be dynamical or the period decrease is caused by the combination of mass transfer and angular momentum loss via magnetic braking. To satisfy such a period decrease, a conservative mass transfer (with no magnetic effect) from the more massive to the less massive component would be of the order dm/dt = 1.12 x 10(-7) M-. per year. The study of Rainger, Bell, & Hilditch ( 1990) has shown that VW Bootis is a marginal-contact system and is in the transition phase from B-type to W Ursae Majoris type. A hot spot on the secondary is also found near the neck of the binary star, indicating a primary-to-secondary mass transfer, which is in good agreement with the long time period decrease of the system. These findings suggest that VW Bootis is at the beginning of the overcontact phase. After the formation of a common convective envelope, the system will evolve into a normal overcontact binary.