Mass of BL Lacs from the Velocity Dispersion of the Host Galaxy

We report on new measurements of the stellar velocity dispersion of 4 BL Lac objects for which optical spectroscopy was gathered at the ESO 3.6m using EFOSC2. Assuming the M(BH)--(sigma) relation is valid also for ellipticals hosting active nuclei we estimate the central BH mass of these BL Lacs.