  Variability is a property shared by virtually all active galactic nuclei
(AGNs), and was adopted as a criterion for their selection using data from
multi epoch surveys. Low Luminosity AGNs (LLAGNs) are contaminated by the light
of their host galaxies, and cannot therefore be detected by the usual colour
techniques. For this reason, their evolution in cosmic time is poorly known.
Consistency with the evolution derived from X-ray detected samples has not been
clearly established so far, also because the low luminosity population consists
of a mixture of different object types. LLAGNs can be detected by the nuclear
optical variability of extended objects. Several variability surveys have been,
or are being, conducted for the detection of supernovae (SNe). We propose to
re-analyse these SNe data using a variability criterion optimised for AGN
detection, to select a new AGN sample and study its properties. We analysed
images acquired with the wide field imager at the 2.2 m ESO/MPI telescope, in
the framework of the STRESS supernova survey. We selected the AXAF field
centred on the Chandra Deep Field South where, besides the deep X-ray survey,
various optical data exist, originating in the EIS and COMBO-17 photometric
surveys and the spectroscopic database of GOODS. We obtained a catalogue of 132
variable AGN candidates. Several of the candidates are X-ray sources. We
compare our results with an HST variability study of X-ray and IR detected
AGNs, finding consistent results. The relatively high fraction of confirmed
AGNs in our sample (60%) allowed us to extract a list of reliable AGN
candidates for spectroscopic follow-up observations.
