4 Conclusions

We have shown that the mid-infrared continuum (between 2.4-12 $\mu$m) of Cygnus X-3 in quiescence can be explained by the free-free emission of an expanding wind in the intermediate case between optically thick and optically thin regimes. The low quiescent luminosity of the object in the mid-infrared allows only detection of an upper limit of 15 percent on the possible 4.8 h orbital modulation. A line at 4.3 $\mu$m is detected at a confidence level of more than 4.3 $\sigma$, and is interpreted as the expected HeI (3p-3s) emission line. The close match between the mid-infrared brightness and spectral energy distribution of Cygnus X-3 in quiescence, the HeI emission line, the high mass loss rate in the wind and that of the colliding-wind Wolf-Rayet system WR 147, is consistent with a Wolf-Rayet like companion of WN8 type to the compact object in Cygnus X-3, a later type than suggested by previous works (van Kerkwijk et al. 1996; Fender et al. 1999; Hanson et al. 2000).

Acknowledgements

We warmly thank the ISO project and the ISOCAM and ISOPHOT Teams in Villafranca, Saclay and Heidelberg. We express our gratitude to R. Ogley and to R. Fender for helpful comments, to G. Pooley for giving us the Ryle telescope data and to J. L. Starck for very useful discussions on data analysis. We thank the referee J. Martí for helpful comments on the manuscript. This research has made use of data from the Green Bank Interferometer, a facility of the National Science Foundation operated by the NRAO in support of NASA High Energy Astrophysics programs, of data which were generated by the CGRO BATSE Instrument Team at the Marshall Space Flight Center (MSFC) using the Earth occultation technique, and of quick-look results provided by the ASM/RXTE team. P.A. acknowledges the support of a Hungarian science grant.