to tail-off at faint magnitudes due to the finite dimension of the halo.

A second family is constituted of the subdwarfs of spectral type OB (sd OB). Their evolutionary status is still somewhat uncertain but they are usually associated to the Extended Horizontal Branch and to the evolution thereof (see Caloi, 1989), although some authors refer to some of them as being post-AGB objects. We represent them by Kurucz models with $T_{\rm eff}$ between $20000\,$K and $50000\,$K (see Conlon et al. 1991 but also Table 1 of Lenz et al. 1998), $\log~g~=~5$ and solar metallicity.

Finally, the third family is made of degenerate stars, the so-called white dwarfs. Some models of degenerate stars exist but only a few have their emergent flux published. As a first approximation, degenerate stars are known to have $U\!BV$ colours very similar to black bodies. We therefore computed the colours of black bodies. In any case, this characteristic is perhaps not general and does not apply to the UV part of the spectrum. We finally used the emergent fluxes of the pure-hydrogen atmosphere models for degenerate stars of Koester (1999). We restricted ourselves to effective temperatures ranging from $7000$ K to $80000$ K and to 