Wolf-Rayet stars with compact companions.

The discovery of OB binary X-ray sources during the Uhuru era has led to significant advances with respect to an understanding of massive binary-star evolution with mass exchange and mass loss. One outcome of this was the prediction of a second WR binary phase in the evolution of massive binaries. The probability that the WR plus c (c denotes a compact star, remnant of the supernova) system remains bound is high, since the less massive star explodes. It is pointed out that in a sense, the WR plus c phase was a missing link; all preceding phases were known to exist, as well as the following phase of runaway pulsars, even binary in some cases. It is concluded that among a single-line sample considered there exists a significant fraction of runaway WR stars, which were accelerated most likely by the recoil of a supernova explosion in a close binary prior to the second WR phase. The secondary could be a neutron star or a black hole companion.