NEAR-CONTACT BINARIES WITH MASS TRANSFER: V473 CASSIOPEIAE AND II PERSEI

Abstract

Photometric solutions of two short period close binaries, V473 Cas and II Per, are derived with the 2003 version Wilson-Van Hamme code. It is shown that V473 Cas is a semidetached system with the primary component or the secondary component at its critical Roche Lobe, while II Per is a marginal contact binary system with both components filling their critical Roche Lobes, but with a large temperature difference between the two components. Both systems belong to near-contact binaries with the light curves enhanced around the left shoulder of secondary minimum, which can be explained by hot spots on the secondary components due to the mass transfer via a stream hitting the facing surface of the secondary components. The orbital period investigations based on all available times of light minimum show a secular period existing in these two targets decreasing at the rate of dP/dt = -7.61 x 10(-8) days yr(-1) for V473 Cas and dP/dt = -7.54 x 10(-8) days yr(-1) for II Per. The decrease of the orbital period can be the result of mass transfer from the primary component to the secondary one, which is consistent with the asymmetric light curves of V473 Cas and II Per. Therefore, V473 Cas and II Per are other examples displaying evidence of mass transfer between the two components. We have collected NCBs with secular period variation and find that almost all SD1-type NCBs show decreasing periods and enhancing luminosity on the left shoulder of secondary minimum in their light curves. So the semidetached configuration with the lobe filling primary is more plausible for V473 Cas. Furthermore, the detected enhancing amplitude of luminosity for SD2-type NCBs is smaller than other types of NCBs, which implies that the mass transfer from the primary component to the secondary one is stronger than the transfer in opposite direction. With the orbital period decrease, V473 Cas and II Per will evolve into overcontact binaries with true thermal contact.