GRAVITATIONAL LENSING OF Q-0957- A STUDY OF THE CLUSTER AT Z=0.355(561 )

Redshift measurements for 38 objects as well as VRI photometric data f or similar to 200 objects have been obtained in the field of the multi ply imaged QSO 0957+561. The data analysis leads to the identification of a cluster at < z > = 0.355 surrounding the central lens galaxy G1, with a velocity dispersion 715 +/- 130 km/s. This cluster is identifi ed as a poor to medium-rich cluster, dominated by a giant elliptical g alaxy (G1), and with a large fraction of late type galaxies. Another g roup or cluster, located at about 2' from G1 and centered on G5, is id entified at a redshift of 0.5, in accordance with previous results. Th e high degree of contamination along the line-of-sight is also examine d. The consequences of these observational results for the modeling of the lens and the evaluation of the Hubble constant H-0 are briefly di scussed.