  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.
