VLT multi-epoch radial velocity survey toward NGC 6253. Analysis of
  three transiting planetary candidates
  We measured the radial velocity of 139 stars in the region of NGC 6253,
discussing cluster's membership and binarity in this sample, complementing our
analysis with photometric, proper motion, and radial velocity data available
from previous studies of this cluster, and analyzing three planetary transiting
candidates we found in the field of NGC 6253. Spectra were obtained with the
UVES and GIRAFFE spectrographs at the VLT, during three epochs in August 2008.
The mean radial velocity of the cluster is -29.11+/-0.85 km/s. Using both
radial velocities and proper motions we found 35 cluster's members, among which
12 are likely cluster's close binary systems. One star may have a sub-stellar
companion, requiring a more intensive follow-up. Our results are in good
agreement with past radial velocity and photometric measurements. Furthermore,
using our photometry, astrometry and spectroscopy we identified a new sub-giant
branch eclipsing binary system, member of the cluster. The cluster's close
binary frequency at 29% +/- 9% (34% +/-10% once including long period
binaries), appears higher than the field binary frequency equal to (22% +/- 5%,
though these estimates are still consistent within the uncertainties. Among the
three transiting planetary candidates the brightest one (V=15.26) is worth to
be more intensively investigated with higher percision spectroscopy. We
discussed the possibility to detect sub-stellar companions (brown dwarfs and
planets) with the radial velocity technique (both with UVES/GIRAFFE and HARPS)
around turn-off stars of old open clusters [abridged].
