to as [*giants*]{} have $\log~g~=~3.5$ (Allen, 1976), those we refer to as [*metal poor stars*]{} have $[\rm Fe/H]~=~-1.5$, typical for the halo of our Galaxy. The temperature interval between two adjacent Kurucz models varies along the tracks: it is $250~$K below $10000$ K, $500$ K between $10000$ K and $13000$ K, $1000$ K between $13000$ K and $35000$ K, and $2500$ K between $35000$ K and $50000$ K.

Throughout the rest of this paper, the MS spectrum with $T_{\rm eff}~=~9500\,$K was attributed a magnitude of zero in all filters.

All colour transformations established on the basis of the Kurucz spectra were compared with their equivalent obtained with the stellar spectral atlas prepared by Fioc & Rocca-Volmerange (1997, hereafter FRV). This atlas comprises 65 spectra of stars of various luminosity classes (3 supergiants, 30 giants and 32 dwarfs) and spectral types ($T_{\rm eff}~\in~[2500,180000]$ K). It was constructed from observed spectra whenever possible (Gunn & Strycker 1983; Heck et al. 1984), from synthetic spectra otherwise (Kurucz 1992; Clegg & Middlemass 1987).

The stellar locus for the quasar fields
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In Sect. 6, we analyse the ability of the XMM-OM photometric system to discriminate between quasars and stars. Therefore, we have to delimit 