Mass Exchange in Binary Stars

The semidetached eclipsing binaries (similar to Algol) seem to contradict the theory of stellar evolution, since the less massive components appear to be more evolved.Besides, they are overluminous, fill their critical Roche lobes, and tend to lose mass.All these facts can be eralained in terms of a largescale mass exchange which interchanged the rofes of the components.This successful interpretation of the Algol systems shows that the Roche limit plays an important role in the evolution of binary stars.If so, then mass exchange must occur in practically all eclipsing and spectroscopic binaries, and perhaps even in some visual binaries.The outcome of the process depends mainly on the physical constitution of the mass-losing component.Its final stage is a helium star or a white dwarf.It is hoped that mass exchange can explain a number of peculiar objects such as the Wolf-Rayet stars, novae, U Geminorum stars, and shell stars-or at least some of them.