  We use deep multi-colour (UBVRIJK) images mostly taken in the framework of
the ESO Deep Public Survey (DPS) to optically identify and derive photometric
redshifts for a complete sample of 131 radio sources with S>0.4 mJy, observed
at both 1.4 and 5 GHz as part of the ATESP radio survey. The availability of
multi-wavelength radio and optical information is exploited to infer the
physical properties of the faint radio population. In particular we find that,
considering both early-type galaxies and quasars as sources with an active
nucleus, AGNs largely dominate our sample sub-mJy sample (78%). Further
radio/optical analysis of such AGN component has revealed a somewhat unexpected
class of flat/inverted-spectrum sources with low radio-to-optical ratios
(R<100), which are preferentially identified with early-type galaxies. Such
sources are quite compact (d<10-30 kpc), suggesting core-dominated radio
emission triggered by low luminosity AGNs. This intriguing class of objects
deserves further analysis, and new higher resolution radio observations are
currently under way. In parallel we are developing radio source models, for
both the AGN and the star-forming components of the sub-mJy radio pupulation.
Here we discuss the first results.
