Published online by Cambridge University Press: 14 August 2015
A large part of the stars of the galactic disk are binaries. Abt and Levy (1976,1978) estimate that more than 50% of B type stars are members of binary or multiple systems, and about 50% are close binaries. According to Conti et al. (1980) the total fraction of certain or probable binaries among O stars is about 36 percent. This means that during the evolution their components can fill their Roche volumes, and consequently have to lose mass in order to keep the star within the allowed surface. Several review papers dealing with the evolution of binaries were published recently: Paczynski (1971,1980), van den Heuvel (1976), de Loore (1980,1981), Webbink (1979).