  (Abridged) This paper is the fourth of a series reporting the results of the
ATESP 1.4 GHz radio survey. The survey covers a region of ~26 sq. degrees at
decl. -40 degr. and has produced a catalogue of 2967 radio sources down to ~0.5
mJy (6sigma). In this paper we present the optical identifications over a 3 sq.
degr. region of the public ESO Imaging Survey. In this region deep photometry
and 95% complete object catalogues in the I band are available down to I~22.5.
These data allowed us to identify 57% of the ATESP sources present in the
region (219/386). For a magnitude limited sample of 70 identified sources with
I<19.0 we have obtained spectroscopy at the ESO 3.6 m telescope. This data
allowed us to get redshifts and spectral classification for all sources (except
one). From the analysis of the spectroscopic sample we find that the
composition of the faint radio population abruptly changes going from mJy to
sub-mJy fluxes: the early type galaxies largely dominate the mJy population
(60%), while star forming processes become important in the sub-mJy regime.
Starburst and post-starburst galaxies go from 13% at S>1 mJy to 39% at S<1 mJy.
Nevertheless, at sub-mJy fluxes, early type galaxies still constitute a
significant fraction (25%) of the whole population. We also show that, due to
the distribution of radio to optical ratios, sub-mJy samples with fainter
spectroscopic follow-ups should be increasingly sensitive to the population of
early type galaxies, while a larger fraction of star-forming galaxies would be
expected in microJy samples. We compare our results with others obtained from
studies of sub-mJy samples and we show how the existing discrepancies can be
explained in terms of selection effects.
