A study of the blue straggler population of the old open cluster
  Collinder 261
  Blue Stragglers are stars located in an unexpected region of the
color-magnitude diagram of a stellar population, as they appear bluer and more
luminous than the stars in the turnoff region. They are ubiquitous, since they
have been found among Milky Way field stars, in open and globular clusters, and
also in other galaxies of the Local Group. Here we present a study on the blue
straggler population of the old and metal-rich open cluster Collinder 261,
based on Gaia DR2 data and on a multi-epoch radial velocity survey conducted
with FLAMES@VLT. We also analyze the radial distribution of the blue straggler
population to probe the dynamical status of the cluster. Blue straggler
candidates were identified first with Gaia DR2, according to their position on
the CMD, proper motions, and parallaxes. Their radial distribution was compared
with those of the main sequence, red giant, and red clump stars, to evaluate
mass segregation. Additionally, their radial velocities (and the associated
uncertainties) were compared with the mean radial velocity and the velocity
dispersion of the cluster. When possible, close binaries and long-period
binaries were also identified, based on the radial velocity variations for the
different epochs. We also looked for yellow stragglers, i.e., possible evolved
blue stragglers. We found 53 blue stragglers members of Collinder 261, six of
them already identified in previous catalogs. Among the blue straggler
candidates with radial velocity measurements, we found one long-period binary,
five close-binary systems, three non-variable stars; we also identified one
yellow straggler.
