  Multiwavelength observations of the young massive cluster Westerlund 1 have
revealed evidence for a large number of OB supergiant and Wolf-Rayet binaries.
However, in most cases these findings are based on secondary binary
characteristics such as hard X-ray emission and/or non-thermal radio spectra
and hence provide little information on binary properties such as mass ratio
and orbital period. To overcome this shortcoming we have initiated a long
temporal baseline, VLT/FLAMES+GIRAFFE multi-epoch radial velocity survey that
will provide the first direct constraints on these parameters. This study
presents first results from twenty of the most luminous supergiant stars in
Wd1. Statistically significant radial velocity changes are detected in ~60% of
targets. W43a is identified as a short-period binary, while W234 and the
newly-identified cluster member W3003 are probable binaries and W2a is a strong
binary candidate. The cool hypergiants W243 and W265 display photospheric
pulsations, while a number of early-mid B supergiants display radial velocity
changes that we cannot distinguish between orbital or photospheric motion in
our initial short-baseline survey. When combined with existing observations, we
find 30% of our sample to be binary (6/20) while additional candidate binaries
support a binary fraction amongst Wd1 supergiants in excess of ~40% [ABRIDGED]
