  We present Very Large Telescope optical spectroscopy of nine BL Lac objects
of unknown redshift belonging to the list of optically selected radio loud BL
Lacs candidates. We explore their spectroscopic properties and the possible
link with gamma ray emission. From the new observations we determine the
redshift of four objects from faint emission lines or from absorption features
of the host galaxy. In three cases we find narrow intervening absorptions from
which a lower limit to the redshift is inferred. For the remaining two
featureless sources, lower limits to the redshift are deduced from the very
absence of spectral lines. A search for gamma counterpart emission shows that
six out of nine are Fermi gamma-ray emitters with two new detections. Our
analysis suggests that most of the BL Lac still lacking of redshift information
are most probably located at high redshift.
