A NEW STRAIGHT ARC DETECTED IN A CLUSTER OF GALAXIES AT Z = 0.423

We report on the discovery of a blue arc north of the brightest galaxy (G1) in the cluster of galaxies C10302 + 1658. Extending over more th an 10" for a curvature radius larger than 1 arcmin, this arc A1 is the third "straight" arc discovered so far. A smaller arc A2 (8" long) is embedded in the southern envelope of G1, but is certainly not a count er arc of A1. A somewhat bluer and fainter arclet lies close to the A1 western end. The B-R and R-I color indices of A1 indicate that its re dshift is likely to range around z = 0.8, while the shape and the high er surface brightness of A2 point to a lower redshift. We give the B, R and I photometry of a sample of 36 galaxies in the cluster center, c omplete to the limiting magnitude B = 25. Four spectra of member galax ies confirm the cluster redshift z = 0.424 given by Stocke et al. (199 1). A model of the gravitational lensing configuration reproduces quit e well the shape of both arcs, assuming two sources at redshift z = 0. 8 for A1, and 0.6 for A2. It includes the gravitational effects of the whole cluster (velocity dispersion 1100 km s-1) and of the second bri ghtest galaxy G2 (velocity dispersion 450 km s-1).