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Context. The study of BL Lac objects (BLL) detected in gamma-rays givesinsights on the acceleration mechanisms in play in such systems and is also avaluable tool to constrain the density of the extragalactic background light.As their spectra are dominated by the non-thermal emission of the jet and thespectral features are weak and narrow in the optical domain, measuring theirredshift is challenging. However such a measure is fundamental as it allows afirm determination of the distance and luminosity of the source, and thereforea consistent model of its emission. Aims. Measurement of the redshift of BLLdetected in gamma-rays and determination of global properties of their hostgalaxies. Methods. We observed a sample of eight BLL (KUV 00311-1938, PKS0447-439, PKS 0301-243, BZB J0238-3116, BZB J0543-5532, BZB J0505+0415, BZBJ0816-1311 and RBS 334) with the X-shooter spectrograph installed at the ESOVery Large Telescope in order to take advantage of its unprecedented wavelengthcoverage and of its resolution about 5 times higher than generally used in suchstudies. We extracted UVB to NIR spectra that we then corrected for telluricabsorption and calibrated in flux. We systematically searched for spectralfeatures. When possible, we determined the contribution of the host galaxy tothe overall emission. Results. Of the eight BLL, we measured the redshift offive of them and determined lower limits for two through the detection ofintervening systems. All seven of these objects have redshifts greater than0.2. In two cases, we refuted redshift values reported in other publications.Through careful modelling, we determined the magnitude of the host galaxies. Intwo cases, the detection of emission lines allowed to provide hints on theoverall properties of the gas in the host galaxies.

Spectroscopy of High Energy BL Lac Objects with X-shooter on the VLT