An extensive radial velocity survey toward NGC 6253
  The old and metal rich open cluster NGC 6253 was observed with the FLAMES
multi-object spectrograph during an extensive radial velocity campaign
monitoring 317 stars with a median of 15 epochs per object. All the targeted
stars are located along the upper main sequence of the cluster between 14.8 $<$
V $<$ 16.5. Fifty nine stars are confirmed cluster members both by radial
velocities and proper motions and do not show evidence of variability. We
detected 45 variable stars among which 25 belong to NGC 6253. We were able to
derive an orbital solution for 4 cluster members (and for 2 field stars)
yielding minimum masses in between $\sim$90 M$\rm_J$ and $\sim$460 M$\rm_J$ and
periods between 3 and 220 days. Simulations demonstrated that this survey was
sensitive to objects down to 30 M$\rm_J$ at 10 days orbital periods with a
detection efficiency equal to 50%. On the basis of these results we concluded
that the observed frequency of binaries down to the hydrogen burning limit and
up to 20 days orbital period is around (1.5$\pm$1.3)% in NGC 6253. The overall
observed frequency of binaries around the sample of cluster stars is
(13$\pm$3)%. The median radial velocity precision achieved by the GIRAFFE
spectrograph in this magnitude range was around $\sim$240m$\rm\,s^{-1}$
($\sim$180 m$\rm\,s^{-1}$ for UVES). Based on a limited follow-up analysis of 7
stars in our sample with the HARPS spectrograph we determined that a precision
of 35 m $\rm s^{-1}$ can be reached in this magnitude range, offering the
possibility to further extend the variability analysis into the substellar
domain. Prospects are even more favourable once considering the upcoming
ESPRESSO spectrograph at VLT.
