On the radii of accreting main sequence stars.

The paper examines the behavior of a star that is accreting matter spherically symmetrically at a rather high rate, as is expected for the secondary in a close binary system during the first phase of mass exchange. The increase in the radius of the secondary star during mass exchange is investigated by computing the evolution of a series of stellar models characterized by hydrostatic equilibrium and different (constant) accretion rates. The results show that the increase in the volume of the secondary due to accretion gives rise to a critical period for any close binary configuration in the sense that the components of systems with an initial period smaller than the critical one will come into contact during mass exchange while the components of systems with longer periods will not come into contact. Critical periods are plotted as a function of secondary mass for primaries of 10, 15, and 20 solar masses.