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.