CYGNUS-X-3 AS A BENCHMARK FOR FUNDAMENTAL PROPERTIES OF WOLF-RAYET STARS

The recent IR discovery by van Kerkwijk and collaborators of a massive WN7 star in the unusual X-ray source Cyg X-3 has important, direct implications for the properties of WR stars. The short 4.8 hr orbital period leads to a small WR photospheric core radius (<3-6 R.) with high temperature (>70-90 kK) for a generous range of Population I WR masses, M(WR) approximately 10-50 M.. These values are compatible with those expected for a massive helium star, as predicted by evolutionary models. They are in contradiction with most current wind models, which yield larger radii and lower temperatures. It is argued that with Cyg X-3 one is lucky to have caught at least one system during the rapid spiral-in phase of a close WR + compact binary. It may eventually lead to a longer-lived, hence more common Thorne-Zytkov-like object, in which the compact companion has been completely engulfed. Candidates for such bizarre objects are speculated to be the unusual WN8 stars.