ESO VLT Optical Spectroscopy of BL Lac Objects IV. New spectra and
  properties of the full sample
  We present the last chapter of a spectroscopy program aimed at deriving the
redshift or a lower limit to the redshift of BL Lac objects using medium
resolution spectroscopy. Here we report new spectra for 33 BL Lac object
candidates obtained in 2008-2009 confirming the BL Lac nature of 25 sources and
for 5 objects we obtained new redshifts. These new observations are combined
with our previous data in order to construct a homogeneous sample of \sim 70 BL
Lacs with high quality spectroscopy. All these spectra can be accessed at the
website http://www.oapd.inaf.it/zbllac/. The average spectrum, beaming
properties of the full sample, discussion on intervening systems and future
perspectives are addressed.
