ESO VLT optical spectroscopy of BL Lacertae objects. III. An extension of the sample

We present results of an ongoing program at the ESO VLT for spectroscopy of BL Lac objects lacking a firm redshift estimate and here we report on 15 objects. For 11 sources we confirm the BL Lac classification, and we determine new redshifts for 3 objects, 1 with weak emission lines (PKS 1057-79, z=0.569) and 2 with absorptions from the host galaxy (RBS 1752, z=0.449; RBS 1915, z=0.243); moreover a sub Damped Lyman Alpha (sub-DLA) system is detected in the direction of the BL Lac PKS 0823-223 (z>=0.911). For the remaining 8 BL Lacs, from the very absence of absorption lines of the host galaxy, lower limits to the redshift are deduced with z_min in the interval 0.20-0.80. The remaining three sources are reclassified as a FSRQ (PKS 1145-676, z=0.210; TXS 2346+052, z=0.419) and a misclassified galactic star (PMNJ 1323-3652).