  We selected a sample of sources with high X-ray over optical flux ratio from
the ROSAT PSPC archives. Out of the 16 sources in our sample, one is a
(possibly extreme) BL Lac and two are AGNs. Six more sources are tentatively
classified as AGNs on the basis of their X-ray properties and of our optical-IR
imaging. For one of the AGNs and for the AGN-like blanks we find a discrepancy
between the lack of absorption indicated by the X-ray spectra and the presence
of red counterparts in their error circles. We discuss various possibilities to
reconcile the X-ray and optical information.

Astrophysics