6 Conclusions

As previous WR oriented photometric systems, the combination of filters presented here allows a good discrimination between WR stars and stars of other spectral types, as well as an easy distinction between the WN and the WC stars. In addition to this primary classification, it has been shown that the system proposed here has a much deeper discriminating power. This is particularly true for the WN subtypes for which a separation between WN8, WN7, WNEw and WNEs is achieved. Even if less discriminating power exists for the WC stars, WC9 stars, single WC7-8 stars and WCE stars are easily identified.

An examination of the plots as well as various simulations show that not only the WN/WC distinction but also the separation between the WN subtypes is basically maintained even when the light of the WR star is diluted by that of a possible companion, the only exception being the low-luminosity early subtypes.

The separation between the WN8 stars and the WN7 stars as shown in Fig. 3 is particularly interesting since recent works on the filiations between the WR subtypes indicate that such a discrimination is physically important. Indeed, the WN7 stars are presently considered to be the descendants of objects more massive than $60~M_{\odot}$ while the WN8 stars are descendants of 40 to $60~M_{\odot}$ progenitors (Maeder 1996). Consequently, discriminating these two subclasses can bring information on the mass distribution of the progenitors, i.e. on the initial mass function (IMF) of the observed region.

Combined with state of the art techniques to deconvolve crowded fields, the photometric system discussed here should allow to get information on the upper part of the IMF in the cores of HII regions, well beyond the limit reachable through the usual spectroscopic methods.

Acknowledgements

The authors are indebted to the Fonds National de la Recherche Scientifique (Belgium) for multiple supports. This research is also supported in part by contract ARC94/99-178 "Action de recherche concertée de la Communauté Française'' (Belgium). Partial support through the PRODEX XMM-OM Project is also gratefully acknowledged. We thank our referee Jacques Breysacher for a careful reading of the manuscript.