has a typical scale height of 1 kpc, which corresponds to a distance modulus of 10 magnitudes. This means that disk OBA stars are too bright in apparent magnitudes to be mistaken for quasars. Another possible contaminant is constituted by halo giants. We represent them by Kurucz models with the following parameters: $T_{\rm  eff}~\leq~7000\,    $K, $\log~g~=~3$ and $[\rm Fe/H]~=~-1.5$.

Some hotter stars are also present at high galactic latitudes as trace constituents but since they are bluer, a property they share with low-redshift quasars, they could constitute a strong contaminant in a quasar candidate list. We consider three families. The first one is the Horizontal-Branch (HB) BA stars. These stars have $T_{\rm
eff}~\geq~10000\, $K (up to $\sim  30000\,$K). On the HB, the gravity is strongly dependent on the effective temperature: we chose the dependency law we derived by averaging the ones quoted by Moehler et al. (1999) and by Conlon et al. (1991). According to Moehler et al. (1999), the best fit of Kurucz models to HB star’s spectra is obtained for metal rich chemical compositions. We adopted $[\rm
Fe/H]~=~0.5$. As noticed by Miller & Mitchell (1988), the population of HB stars corresponds to intrinsically bright objects and is observed 