Theoretical characteristics of WR+O binaries.

Using two kinds of stellar evolution models, one with convective cores determined by means of the Schwarzschild criterion, and the other with convective cores enlarged by overshooting, the possible masses of the WR-stars in a number of double-lined WR binaries are discussed. It is shown that the virtual absence of observed main-sequence B-type companions in WR systems may, for mass-transfer dominated evolution, be explained as a result of accretion effects on the inner structure of the companion star (rejuvenation) and, for wind mass-loss dominated evolution, as a result of the large difference in luminosity between the WR star and its companion. Finally, investigating the formation process of the WR-stars in WR + O binaries, it is found that some of these systems must have been formed through mass exchange between the components. Especially this is expected to be the case for the WR binaries in the Small Magellanic Cloud.