AGNs among ROSAT Blank Field Sources
  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. 
I. Cagnoni (1), M. Elvis (2), D.-W. Kim (2), J.-S. Huang (2), A.   Celotti (3) and F. Nicastro (2) ((1) Universita' dell'ins...