The accreting component of mass-exchange binaries

We have studied the effect of rapid accretion on a moderate-mass stellar model. The surface boundary condition is examined for two modes of accretion: disk and direct impact. It is found that the effective temperature must be increased for disk accretion but not for direct-impact accretion. In the latter mode the excess kinetic energy of the transfer stream is radiated in an optically thick hot spot or in a hot equatorial band. The remainder of the star is unaffected by the direct-impact accretion. Evolutionary calculations for direct-impact accretion show that the mass-gaining star swells. The present orbital elements of X-ray binaries indicate that these systems went through a contact phase of evolution. Significant quantities of mass and angular momentum may have been lost at that time