SIMBAD references

Redshift measurements for 38 objects as well as VRI photometric data for ∼200 objects have been obtained in the field of the multiply 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 identified as a poor to medium-rich cluster, dominated by a giant elliptical galaxy (G1), and with a large fraction of late type galaxies. Another group or cluster, located at about 2' from G1 and centered on G5, is identified at a redshift of 0.5, in accordance with previous results. The high degree of contamination along the line-of-sight is also examined. The consequences of these observational results for the modeling of the lens and the evaluation of the Hubble constant H₀ are briefly discussed.