THE WOLF-RAYET COUNTERPART OF CYGNUS-X-3

We present orbital-phase resolved I and K-band spectroscopy of Cygnus X-3. All spectra show emission lines characteristic of Wolf-Rayet star s of the WN subclass. On time scales longer than about one day, the li ne strengths show large changes, both in flux and in equivalent width. In addition, the line ratios change, corresponding to a variation in spectral subtype of WN6/7 to WN4/5. We confirm the finding that at tim es when the emission lines are weak, they shift in wavelength as a fun ction of orbital phase, with maximum blueshift coinciding with infrare d and X-ray minimum, and maximum redshift half an orbit later. Further more, we confirm the prediction - made on the basis of previous observ ations - that at times when the emission lines are strong, no clear wa velength shifts are observed. We describe a simplified, but detailed m odel for the system, in which the companion of the X-ray source is a W olf-Rayet star whose wind is at times ionised by the X-ray source, exc ept for the part in the star's shadow. With this model, the observed s pectral variations can be reproduced with only a small number of free parameters. We discuss and verify the ramifications of this model, and find that, in general, the observed properties can be understood. We conclude that Cyg X-3 is a Wolf-Rayet/X-ray binary.