Redshift measurements of the brightest cluster galaxies of the gravitational lens 0957 + 561

We present the results of a spectroscopic survey of the brightest objects in a field centered on the gravitational lens system 0957 + 561. Redshift measurements have been used to separate field galaxies from members of the lensing cluster at z = 0.355, allowing the determination of a luminosity-weighted center for the cluster and a tentative value for its radial velocity dispersion $$\approx\,550\,\text{km}\,\text{s}^{-1}$$. These data also provide evidence that there may be an additional group or cluster of galaxies at z = 0.503.