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.
Date added: Tue, 8 Oct 13