  BL Lacertae (Lac) objects that are detected at very-high energies (VHE) are
of fundamental importance to study multiple astrophysical processes, including
the physics of jets, the properties of the extragalactic background light and
the strength of the intergalactic magnetic field. Unfortunately, since most
blazars have featureless optical spectra that preclude a redshift
determination, a substantial fraction of these VHE extragalactic sources cannot
be used for cosmological studies. To assess whether molecular lines are a
viable way to establish distances, we have undertaken a pilot program at the
IRAM 30m telescope to search for CO lines in three BL Lac objects with known
redshifts. We report a positive detection of M_H2 ~ 3x10^8 Msun toward 1ES
1959+650, but due to the poor quality of the baseline, this value is affected
by a large systematic uncertainty. For the remaining two sources, W Comae and
RGB J0710+591, we derive 3sigma upper limits at, respectively, M_H2 < 8.0x10^8
Msun and M_H2 < 1.6x10^9 Msun, assuming a line width of 150 km/s and a standard
conversion factor alpha=4 M_sun/(K km/s pc^2). If these low molecular gas
masses are typical for blazars, blind redshift searches in molecular lines are
currently unfeasible. However, deep observations are still a promising way to
obtain precise redshifts for sources whose approximate distances are known via
indirect methods. Our observations further reveal a deficiency of molecular gas
in BL Lac objects compared to quasars, suggesting that the host galaxies of
these two types of active galactic nuclei (AGN) are not drawn from the same
parent population. Future observations are needed to assess whether this
discrepancy is statistically significant, but our pilot program shows how
studies of the interstellar medium in AGN can provide key information to
explore the connection between the active nuclei and the host galaxies.
