ZBLLAC -- A spectroscopic database of BL Lacertae objects
  This paper describes the database of optical spectra of BL Lacertae (BLL)
objects ZBLLAC available at the URL https://web.oapd.inaf.it/zbllac/. At the
present date, it contains calibrated spectra for 295 BLL. For about 35% of
them, we report a firm measure of redshift z while for 35 sources we set a
lower limit on z based on the detection of intervening absorption systems,
mainly ascribed to Mg II (2800 Ang). We report here on the architecture of the
database and on its web front-end that permits to filter, query and
interactively explore the data. We discuss some properties of the objects in
the present dataset by giving the distribution of the redshifts and reporting
on the detected emission lines, which turn out to be mainly forbidden and
ascribed to [O II] (3737 Ang) and [O III] (5007 Ang). Finally, we discuss on
intervening absorption systems detected in 35 BLLs that allow us to set lower
limits to their distance.
