A proper motion study of the globular cluster M55
  We have derived the absolute proper motion (PM) of the globular cluster M55
using a large set of CCD images collected with the du Pont telescope between
1997 and 2008. We find (PM_RA*cos(DEC), PM_DEC) = (-3.31 +/- 0.10, -9.14 +/-
0.15) mas/yr relative to background galaxies. Membership status was determined
for 16 945 stars with 14<V<21 from the central part of the cluster. The PM
catalogue includes 52 variables of which 43 are probable members of M55. This
sample is dominated by pulsating blue straggler stars but also includes 5
eclipsing binaries, three of which are main sequence objects. The survey also
identified several candidate blue, yellow and red straggler stars belonging to
the cluster. We detected 15 likely members of the Sgr dSph galaxy located
behind M55. The average PM for these stars was measured to be (PM_RA*cos(DEC),
PM_DEC)=(-2.23 +/- 0.14, -1.83 +/- 0.24) mas/yr.
