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 im plications for the properties of WR stars. The short 4.8 hr orbital pe riod 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 mode ls. They are in contradiction with most current wind models, which yie ld 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 spir al-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 suc h bizarre objects are speculated to be the unusual WN8 stars.