both the quasar (see Sect. 2.4) and the stellar loci in the multidimensional colour space defined by the XMM-OM system. The stellar locus is defined by all the stars that are potential contaminants of the quasar candidate population. When looking at high galactic latitudes, the field stars are halo main sequence stars. We modelize their colours by integrating the emergent fluxes from Kurucz model atmospheres. We represent halo main sequence stars by the whole range of $T_{\rm   eff}$, $\log~g~=~4$ and $[\rm Fe/H]~=~-1.5$. Due to evolution, only the part for which $T_{\rm  eff}~\leq~7000\,$K (corresponding to an early F spectral type) is still populated. These latter stars are the major constituents of the field stellar population. The reason why we consider the hotter stars as well is that their colours are close to those of other families of stars. We will use that property in Sect. 6, and discuss it further there.

Disk main sequence stars are also a possible contaminant in a list of quasar candidates. However, only the cool end ($T_{\rm
eff}~\leq~7000\,$K) could be a problem and is taken into account. We chose to represent them by a $\log~g~=~4$ and a solar metallicity. Indeed, the disk is rather thin: it 