  A new method is presented for recovering the abundances of Damped Ly alpha
systems (DLAs) taking into account the effects of dust depletion. For the first
time, possible variations of the chemical composition of the dust are taken
into account in estimating the depletions. No prior assumptions on the
extinction properties of the dust are required. The method requires a set of
abundances measured in the gas and a set of parameters describing the chemical
properties of the dust. A large subset of these parameters is determined from
interstellar observations; the others are free parameters for which an educated
guess can be made. The method is able to recover the abundances of the SMC
starting from SMC interstellar measurements apparently discrepant from the
stellar ones. Application of the method to 22 DLAs with available [Fe/H] and
[Si/Fe] measurements gives the following results: (1) the mean metallicity of
the corrected data is <[Fe/H]> ~ -1.0 dex, about 0.5 dex higher than that of
the original data; (2) the slope of the [Fe/H] versus redshift relation is
steeper for the corrected data (m ~ -0.3 dex) than for the original ones (m ~
-0.2 dex); (3) the corrected [Si/Fe] ratios are less enhanced, on average, than
those found in Galactic stars of similar, low metallicity; (4) a decrease of
the [Si/Fe] versus [Fe/H] ratios, expected by "time delay" models of chemical
evolution, is found for the corrected data; (5) the [Si/Fe] ratios tend to
increase with redshift once corrected; (6) consistency between [Si/Fe] and
[S/Zn] measurements, two independent estimators of the alpha/Fe ratio, is found
only for the corrected abundances.
