[en] gravitational lensing ; cosmological parameters ; quasars : general
[en] Aims. We measure the redshift of the lensing galaxy in eight gravitationally lensed quasars in view of determining the Hubble parameter H-0 from the time delay method. Methods. Deep VLT/FORS1 spectra of lensed quasars are spatially deconvolved in order to separate the spectrum of the lensing galaxies from the glare of the much brighter quasar images. A new observing strategy is devised. It involves observations in Multi-Object-Spectroscopy (MOS) which allows the simultaneous observation of the target and of several PSF and flux calibration stars. The advantage of this method over traditional long-slit observations is a much more reliable extraction and flux calibration of the spectra. Results. For the first time we measure the redshift of the lensing galaxy in three multiply-imaged quasars: SDSS J1138+0314 (zlens = 0.445), SDSS J1226-0006 (z(lens) = 0.517), SDSS J1335+0118 (z(lens) = 0.440), and we give a tentative estimate of the redshift of the lensing galaxy in Q 1355- 2257 (z(lens) = 0.701). We confirm four previously measured redshifts: HE 0047-1756 (z(lens) = 0.407), HE 0230-2130 (z(lens) = 0.523), HE 0435-1223 (z(lens) = 0.454) and WFI J2033-4723 (z(lens) = 0.661). In addition, we determine the redshift of the second lensing galaxy in HE 0230-2130 (z(lens) = 0.526). The spectra of all lens galaxies are typical for early-type galaxies, except for the second lensing galaxy in HE 0230-2130 which displays prominent [OII] emission.