The various color diagrams and tests presented here are based on two distinct data sets. The first one consists of observations performed between 1991 and 1993 (some results are reported in Vreux et al. 1996). The second data set consists of spectrophotometric data from Torres-Dodgen & Massey (1988, hereafter TM) for the WR stars and from Jacoby et al. (1984, hereafter JHC) for the other stars.
The observations were conducted at the ESO La Silla observatory during three dedicated runs using the ESO standard one-channel photometer attached to the 1 m telescope. The photometer was equipped with a Quantacon RCA 31034 tube. Data were obtained over a total of 26 nights: 6 in March 91, 13 in February 92 and 7 in September 93. Altogether 45 WR stars and 121 non - WR stars were observed during these observing runs.
The observing strategy was planned so as to fully benefit from the RANBO2 reduction algorithm (Manfroid 1993). The 121 non - WR constant stars were observed frequently in order to determine the photometric parameters. This was specially important because of the occasional presence of fast, large-amplitude, extinction variations due to volcanic aerosols. Except for a zero-point adjustment, the WR photometric system is the instrumental system. Due to the narrow passbands and the stability of the filters, no color corrections were needed between the various runs. The zero of the system is fixed by the A0V star HD 104430 for which the value 6.157 is imposed in every band.
Eight of the WR and most of the non - WR stars were not observed in the c1 filter which we had not yet decided to include in the system. They could thus not be used in the subsequent color diagrams (Sect. 4.2).
The TM spectrophotometric data base is made of 171 spectra of Galactic and Magellanic Clouds WR stars. Some of these spectra could not be used because their wavelength range was not sufficient. Individual inspection also led to the rejection of some spectra due to a too high noise level or to obvious anomalies in the relevant wavelength regions. Finally, we were left with 117 Galactic and Magellanic WR spectra.
The simulations presented in Sect. 5.1 also required the use of normal, i.e. non - WR stars spectra. This has been done through the use of a representative subsample of the JHC catalog, made of 28 stars.
The final data set consists of 129 WR stars. There are 37 galactic WR stars for which photometric data are available in all the needed filters and 117 WR stars for which spectrophotometric data are available (71 galactic and 36 magellanic WR stars). In this sample, there are 25 WR stars for which both photometric and spectrophotometric data are available. Altogether, this sample of 129 WR stars can be considered as highly representative as it covers almost 50% of the whole presently known WR population of our Galaxy and the LMC.
In the present paper, "WR'' followed by a number will refer to a galactic WR star (van der Hucht et al. 1981) while "Brey'' followed by a number will refer to an LMC WR star (Breysacher 1981).
The non - WR sample consists of 64 objects: 36 of the 121 stars mentioned above, ranging from A to K and from luminosity class V to luminosity class III for which photometric observations have been performed, and 28 stars ranging from O to M and from dwarfs to supergiants (including Of stars) for which spectrophotometric data have been used.
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