The Gaia-ESO Survey: Target selection of open cluster stars
  The Gaia-ESO Survey (GES) is a public, high-resolution spectroscopic survey
with FLAMES@VLT. GES targeted in particular a large sample of open clusters
(OCs) of all ages. The different kinds of OCs are useful to reach the main
science goals, which are the study of the OC structure and dynamics, the use of
OCs to constrain and improve stellar evolution models, and the definition of
Galactic disc properties (e.g. metallicity distribution). GES is organised in
19 working groups (WGs). We describe here the work of three of them, WG4 in
charge of the selection of the targets within each cluster), WG1 responsible
for defining the most probable candidate members, and WG6 in charge of the
preparation of the observations. As GES has been conducted before Gaia DR2, we
could not make use of the Gaia astrometry to define cluster members. We made
use of public and private photometry to select the stars to be observed with
FLAMES. Candidate target selection was based on ground-based proper motions,
radial velocities, and X-ray properties when appropriate, and it was mostly
used to define the position of the clusters' evolutionary sequences in the
colour-magnitude diagrams. Targets for GIRAFFE were selected near the sequences
in an unbiased way. We used available information on membership only for the
few UVES stars. We collected spectra for 62 confirmed OCs (a few more were
taken from the ESO archive). Among them are very young clusters, where the main
targets are pre-main sequence stars, clusters with very hot and massive stars
currently on the main sequence, intermediate-age and old clusters where evolved
stars are the main targets. The selection of targets was as inclusive and
unbiased as possible and we observed a representative fraction of all possible
targets, thus collecting the largest, most accurate, and most homogeneous
spectroscopic data set on ever achieved. [abridged]
