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We report on ESO Very Large Telescope optical spectroscopy of 42 BL Lacertaeobjects of unknown redshift. Nuclear emission lines were observed in 12objects, while for another six we detected absorption features due to theirhost galaxy. The new high S/N spectra therefore allow us to measure theredshift of 18 sources. Five of the observed objects were reclassified eitheras stars or quasars, and one is of uncertain nature. For the remaining 18 theoptical spectra appear without intrinsic features in spite of our ability tomeasure rather faint (EW nearly 0.1 Angstrom) spectral lines. For the lattersources a lower limit to the redshift was set exploiting the very fact that theabsorption lines of the host galaxy are undetected on the observed spectra.Comment: 33 pages, 11 figures, accepted for publication on A

ESO Very Large Telescope Optical Spectroscopy of BL Lacertae Objects. II. New Redshifts, Featureless Objects, and Classification Assessments