Spectroscopy of 10 gamma-ray BL Lac objects at high redshift

We present high S/N optical spectra of 10 BL Lac objects detected at GeV energies by Fermi satellite (3FGL catalog), for which previous observations suggested that they are at relatively high redshift. The new observations, obtained at the 10 m Gran Telescopio Canarias, allowed us to find the redshift for J0814.5+2943 (z = 0.703) and we can set spectroscopic lower limit for J0008.0+4713 (z>1.659) and J1107.7+0222 (z>1.0735) on the basis of Mg II intervening absorption features. In addition we confirm the redshifts for J0505.5+0416 (z=0.423) and for J1450+5200 (z>2.470). Finally we contradict the previous z estimates for five objects (J0049.7+0237, J0243.5+7119, J0802.0+1005, J1109.4+2411, and J2116.1+3339).