  We present high-resolution UES spectra of the quasar PHL 957 obtained for
studying the foreground Damped Ly-alpha galaxy at z=2.309. Measurements of
absorption lines lead to accurate abundance determinations of Fe, S and N which
complement measurements of Zn, Cr and Ni already available for this system. We
find [Fe/H]= -2.0 (+/-0.1), [S/H]= -1.54 (+/-0.06) and [N/H]=-2.76 (+/-0.07).
The ratio [Fe/Zn]=-0.44 provides evidence that about 74% of iron and about 28%
of zinc are locked into dust grains with a dust-to-gas ratio of about 3% of the
Galactic one. The total iron content in both gas and dust in the DLA system is
[Fe/H]=-1.4. This confirms a rather low metallicity in the galaxy, which is in
the early stages of its chemical evolution. The detection of SII allows us to
measure the SII/ZnII ratio, which is a unique diagnostic tool for tracing back
its chemical history, since it is not affected by the presence of dust.
Surprisingly, the resulting relative abundance is [S/Zn]=0.0 (+/-0.1), at
variance with the overabundance found in the Galactic halo stars with similar
metallicity. We emphasize that the [S/Zn] ratio is solar in all the three DLA
absorbers with extant data. Upper limits are also found for Mn, Mg, O and P
and, once the dust depletion is accounted for, we obtain [Mg/Fe]_{c} <+0.2,
[O/Fe]_{c}< 0.4, [Mn/Fe]_{c}<+0.0 and [P/Fe]_{c}<-0.7. The [alpha/Fe] values do
not support a Galactic halo-like abundances implying that the chemical
evolution of this young galaxy is not reproducing our own Galaxy's evolution.
