  The optical spectroscopic followup of 27 sources belonging to a sample of 30
high-energy objects selected by positionally cross correlating the first
Fermi/LAT Catalog and the ROSAT All-Sky Survey Bright Source Catalog is
presented here. It has been found or confirmed that 25 of them are BL Lacertae
objects (BL Lacs), while the remaining two are Galactic cataclysmic variables
(CVs). This strongly suggests that the sources in the first group are
responsible for the GeV emission detected with Fermi, while the two CVs most
likely represent spurious associations. We thus find an 80% a posteriori
probability that the sources selected by matching GeV and X-ray catalogs belong
to the BL Lac class. We also show suggestions that the BL Lacs selected with
this approach are probably high-synchrotron-peaked sources and in turn good
candidates for the detection of ultra-high-energy (TeV) photons from them.
