Stability and Pulsations of Wolf-Rayet Stars

The stability of Wolf-Rayet stars is reviewed. According to linear theory, Wolf-Rayet stars are strange - mode unstable with growth rates in the dynamical regime for a wide range of stellar parameters. We emphasize, that the mechanism of strange - mode instability is largely independent of opacity. By following the evolution of strange - mode unstable stellar envelopes into the nonlinear regime the final result of the instability is determined. The instability leads to finite amplitude pulsations which inflate the stellar envelope considerably thus implying an increase of pulsation periods. The final periods are compatible with that observed in WR 123. Velocity amplitudes of the order of 100 km/sec are reached and the mean acoustic energy flux is comparable with the kinetic enemy flux observed in Wolf-Rayet stars.