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Aims. We aim to determine the redshift (or stringent lower limits) of anumber of bright BL Lacs objects. Methods. We secured medium resolution opticaland near-infrared spectra of 4 bright BL Lac objects of unknown redshift usingthe spectrograph X-Shooter at the ESO-VLT. Results. In spite of the highquality of the spectra and the extended spectral range of the observations wehave not detected intrisic spectral features for these sources. However we areable to provide strigent lower limits to their redshift. In particular, for thetwo TeV sources PG 1553+113 and H 1722+119 we infer z > 0.30 and z > 0.35respectively. We also detect an intervening Ca II absorption doublet in thespectrum of MH 2136-428 that is ascribed to the the halo of a nearby giantelliptical galaxy at \sim 100 kpc of projected distance. Conclusions. Under thehypothesis that all BL Lacs are hosted by luminous bulge dominated galaxies,the present state of art spectroscopic observations of bright BL Lacs indicatethat these objects are likely sources with extremely beamed nuclear emission .We present simulations to show under which circustances it will be possible toprobe this hypothesis from the detection of very weak absorptions using thenext generation of extremely large optical telescopes.

Spectroscopy of BL Lac objects of extraordinary luminosity