  We present preliminary results on the optical identifications of sources from
the High Energy Large Area Survey performed with XMM-Newton (HELLAS2XMM). This
survey covers about 3 square degrees of sky down to a 2-10 keV flux of 7 x
10^-15 erg cm-2 s-1 (Baldi & Molendi these proceedings, Baldi et al. 2001). The
survey good sensitivity over a large area allows us to confirm and extend
previous Chandra results about a population of X-ray luminous but optically
dull galaxies emerging at 2-10 keV fluxes of about 10-14 erg cm-2 s-1. Although
the statistics are still rather poor, it appears that at these X-ray fluxes the
fraction of these galaxies is similar or even higher than that of narrow line
AGN.
