Redshift survey of gravitational arclets in Abell 2390
  In this paper, we present new determinations of redshifts on a sample of gravitational arclets identified in the cluster of galaxies Abell 2390. The arclets candidates were selected from their elongated morphology as seen in recent deep HST images of the cluster. The spectra display various features, and the redshifts of the background objects range from 0.398 to 1.268, most of them being between 0.6 and 0.9 with [O II] $\lambda$=3727\AA\ being the main emission line encountered. This distribution is mostly concentrated in two redshifts planes at $z=0.64$ and $z=0.90$. The existence of two pairs of background galaxies at different redshifts, almost superimposed on the same line of sight are used to infer some limits on the mass of galaxy-size deflectors. A detailed writing of the lensing distorsion by two lens planes at different redshifts is developped for this purpose. Finally, [O II] equivalent widths are used to compute approximate star formation rates which appear to be of the same order of magnitude as in nearby spiral galaxies. The implications on both the lensing properties of the cluster and the analysis of the field galaxy evolution are also discussed. 
J. Bezecourt and G. Soucail (Observatoire Midi-Pyrenees, Toulouse)