  We present the results of a site characterization study carried out at the
Astronomical Observatory of the Autonomous Region of the Aosta Valley (OAVdA),
in the Western Italian Alps, aimed at establishing its potential to host a
photometric transit search for small-size planets around a statistically
significant sample of nearby cool M dwarfs. [abridged] we gauged site-dependent
observing conditions such as night-sky brightness, photometric precision, and
seeing properties. Public meteorological data were also used in order to help
in the determination of the actual number of useful observing nights per year.
The measured zenithal $V$-band night-sky brightness is typical of that of very
good, very dark observing sites. The extinction registered at $V$ band is not
dissimilar from that of other sites. The median seeing over the period of in
situ observations is found to be $\sim1.7^{\prime\prime}$. Given the limited
duration of the observations, we did not probe any possible seeing seasonal
patterns, or the details of its possible dependence on other meteorological
parameters, such as wind speed and direction. Moreover, our data show that the
seeing at the observatory was reasonably stable during most of the nights. The
fraction of fully clear nights per year amounts to 39\%, while the total of
useful nights increases to 57\% assuming a (conservative) cloud cover of not
more than 50\% of the night. Based on the analysis of photometric data
collected over the period May-August 2009 for three stellar fields centered on
the transiting planet hosts WASP-3, HAT-P-7, and Gliese 436, we achieve
seeing-independent best-case photometric precision
$\sigma_\mathrm{ph}\lesssim3$ mmag (rms) in several nights for bright stars
($R\lesssim 11$ mag). A median performance $\sigma_\mathrm{ph}\sim6$ mmag
during the observing period is obtained for stars with $R\lesssim13$ mag.
[abridged]
