2 Selection of the filters

Vreux et al. (1990), have shown that the sole use of the equivalent widths of He II $\lambda$ 10124 and He I $\lambda$ 10830 allowed to divide the WN population into six groups which are linked to the "classical'' WN subtypes and to a "temperature scale'' defined by the models of Hamann & Schmutz (1987). It was then suggested that a four-band photometric system (two filters for the above mentioned lines, one for a carbon line such as C III $\lambda$ 9715, and one for a continuum value) could lead to a useful classification system of the WR stars, particularly suited for crowded fields and for regions with a relatively high interstellar absorption.

Unfortunately, at that time, it quickly appeared that our access to an instrumentation suited for the 1 $\mu$m region was very limited. Consequently, a "similar system'' using filters operating in the classical visible region was designed. Three "line filters'' were selected, centered on He II $\lambda$ 4686, He I $\lambda$ 5876 and C IV $\lambda\lambda$ 5801-5812 respectively.

The choice of the first two filters is obvious if one refers to the paper of Conti et al. (1990) where it is shown that an excellent correlation exists between, respectively, He II $\lambda$ 10124 and He II $\lambda$ 4686, and between He I $\lambda$ 10830 and He I $\lambda$ 5876.

The C IV line has been preferred to the much stronger nearby C III $\lambda$ 5696 line because, in the same reference, it is shown that the C III $\lambda$ 5696 line exhibits a "peculiar behaviour'' relative to spectral types.

Since the three "line filters'' are spread over about 1200 Å, it is necessary to have two "continuum filters''. As is well known, finding an access to the continuum in a WR spectrum is not an obvious task. Here we have chosen two filters centered on $\lambda$ 5057 Å and $\lambda$ 6051 Å respectively. The first one avoids the [O III] $\lambda\lambda$ 4959-5007 nebular lines which could have been a cause of contamination in our diagrams. An a posteriori justification of the latter is given by Koesterke & Hamann (1995) who consider that this wavelength gives the first access to a relatively clean "continuum window'' in the WC stars, other windows being located at longer wavelengths.

The main characteristics of the filters discussed here are given in Table 1.

  

Table 1: Main characteristics of the filters