  We present orbital-phase resolved I and K-band spectroscopy of Cygnus X-3.
All spectra show emission lines characteristic of Wolf-Rayet stars of the WN
subclass. On time scales longer than about one day, the line 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 times when the emission lines are weak,
they shift in wavelength as a function of orbital phase, with maximum blueshift
coinciding with infrared and X-ray minimum, and maximum redshift half an orbit
later. Furthermore, we confirm the prediction -- made on the basis of previous
observations -- that at times when the emission lines are strong, no clear
wavelength shifts are observed. We describe a simplified, but detailed model
for the system, in which the companion of the X-ray source is a Wolf-Rayet star
whose wind is at times ionised by the X-ray source, except for the part in the
star's shadow. With this model, the observed spectral 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.
