ar X iv : a st ro - p h / 05 10 46 4 v 2 2 A ug 2 00 6 1 Evolutionary Status of Late - Type Contact

The old model of an unevolved, cool contact binary, in which the secondary component is strongly oversized due to energy transfer from the primary, and the whole system is out of thermal equilibrium, encounters serious problems. I present a new scenario for evolution of contact binaries, which solves the problem of thermal nonequilibrium by assuming that contact binaries are past mass exchange with a mass ratio reversal. The scenario is divided into three phases. In Phase I loss of angular momentum (AM) due to magnetized wind of a detached binary is followed until the primary component fills its critical Roche lobe. In Phase II mass transfer takes place until mass ratio reversal. Arguments are given in favor of such a process in pre-contact binaries. In Phase III an approximate evolutionary path of the contact binary is followed until a possible coalescence. AM loss, evolutionary effects of the components and mass transfer to the primary are taken into account. It is concluded that W UMa type binaries are old objects with secondaries in an advanced evolutionary stage, possibly with small helium cores. Both components fulfill the mass-radius relation for contact binaries while being in thermal equilibrium.