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Using stellar evolution models of two different kinds, one with convective cores determined by the Schwarzschild criterion, one with enlarged convective cores, we discuss the masses of the WR stars in a number of double-lined WR binaries. For the main group of systems the mass of the WR-star is between 10 and 20 M.

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Evolutionary computations are presented for massive stars between 20 M 0 and 100 M 0 with chemical abundances holding for the Small Magellanic Cloud, i.e. X = .76 and Z = .003. Mass loss by stellar wind is taken into account during core hydrogen burning. After core hydrogen burning some models are considered as members of close binary systems and are followed during their Roche lobe overflow stage according an early case B of mass transfer. During the core helium burning stage of the RL0F remnants mass loss rates comparable to WR stars are included in order to study the formation and the evolution of WR stars.Comparison with similar galactic computations (Vanbeveren, Packet, 1978) is made. CORE HYDROGEN BURNINGWe used the mass loss rate formalism of de Loore et al. (1977Loore et al. ( , 1978, i.e. . M = -NL/c^In order to make direct comparison with galactic models possible the traditional values of N (=0,100,300 and 500) were considered. The comparison revealed that the luminosity in SMC models is 5 to 10% higher at the zero age main sequence than in corresponding galactic models. At redpoint the difference is less than 5%. The effective temperatures in the SMC computations are about 10% higher. This results in a shift of the main sequence band to the blue by about one subclass. This is shown in Figure 1. The stellar radii are 20 to 30% smaller at ZAMS and 20% at redpoint. Central temperatures are 10% higher, and central densities 30 to 40% higher in the SMC. This effect causes a shortening of the hydrogen main sequence lifetime and thus a faster nuclear burning occurs. In the SMC, this shortening is largely compensated by a larger amount of hydrogen (X-j n -jt = .76); it follows that the MS-lifetime is about 10 to 15% larger in the SMC than in the Galaxy. Furthermore the behaviour of the convective core, the MS end mass, the mass-luminosity relation, the M rates for equal values of N differ by less than 10% from galactic results.