  We present a spectroscopic and imaging study of the z=0.558 MgII absorption
system towards the BL Lac PKS 0118-272. At the absorber redshift we detect
lines of the low ionization species MgI, MgII, CaII, TiII, MnII, and FeII. The
column densities of these species are typical of the interstellar gas in the
disk of the Galaxy. By assuming dust-free gas with solar abundances we infer
N(HI) > 2.5 x 10^19 cm^-2. The high value of this conservative lower limit
indicates that the absorber is a Damped Ly alpha system. We find [Ti/Fe] =
+0.3, in agreement with [Ti/Fe] measurements in DLA absorbers, but [Mn/Fe] =
+0.4, at variance with the values [Mn/Fe]<0 common to DLA systems. The measured
[Ti/Fe] and [Mn/Fe] ratios match remarkably well the differential depletion
pattern of low-density interstellar clouds in the Galaxy. Unlike high-redshift
DLA systems (z>2), the z=0.558 absorber seems to originate in a galaxy that has
already attained the abundances and dust content of present-day disk galaxies.
The analysis of our imaging data lends support to the presence of an
intervening galaxy. After careful subtraction of the BL Lac image, an object at
1.6 arcsec from PKS 0118-272 is detected. At the absorber redshift the
projected distance (14 h_50^-1 kpc) and the absolute magnitude (M_R ~ -22.3) of
this companion are consistent with those found for galaxies associated with
low-redshift DLA systems.
