  Aims: The knowledge of the redshift of a lensing galaxy that produces
multiple images of a background quasar is essential to any subsequent modeling,
whether related to the determination of the Hubble constant H_0 or to the mass
profile of the lensing galaxy. We present the results of our ongoing
spectroscopic observations of gravitationally lensed quasars in order to
measure the redshift of their lensing galaxies. We report on the determination
of the lens redshift in seven gravitationally lensed systems.
  Methods: Our deep VLT/FORS1 spectra are spatially deconvolved in order to
separate the spectrum of the lensing galaxies from the glare of the much
brighter quasar images. Our observing strategy involves observations in
Multi-Object-Spectroscopy (MOS) mode which allows the simultaneous observation
of the target and of several crucial PSF and flux calibration stars. The
advantage of this method over traditional long-slit observations is that it
allows a much more reliable extraction and flux calibration of the spectra.
  Results: We obtain the first reliable spectra of the lensing galaxies in six
lensed quasars: FBQ 0951+2635 (z=0.260), BRI 0952-0115 (z=0.632), HE 2149-2745
(z=0.603), Q 0142-100 (z=0.491), SDSS J0246-0825 (z=0.723), and SDSS J0806+2006
(z=0.573). The last 3 redshifts also correspond to the MgII doublet seen in
absorption in the quasar spectra at the lens redshift. Our spectroscopic
redshifts of HE 2149-2745 and FBQ 0951+2635 are higher than previously
reported, which means that H_0 estimates from these two systems must be revised
to higher values. Finally, we reanalyse our spectra of Q 1355-2257 and find
MgII in absorption at z=0.702, confirming our previous redshift estimate. The
spectra of all lenses are typical of early-type galaxies.
