Massive binaries in Westerlund 1. VIII.

We identify 20 new OB I-III binaries, a WN9h binary, and a WC9d binary, greatly increasing the number of directly confirmed binary systems in Westerlund 1, while 12 O9-9.5 Iab-III stars are identified as candidate binaries based on radial velocity changes that are inconsistent with photospheric variability. The 173.9 day SB1 W1030 represents the first longer-period system identified in the cluster, while the determination of a 53.95 day period for W44 (WR L) makes it the first Wolf-Rayet binary in Westerlund 1 with a confirmed orbital period greater than ten days.

Our results suggest the binary fraction in the OB population is at least ~40%, and may be significantly higher. These results demonstrate that binary systems can be effectively identified in the population of OB I-III stars evolving off the main sequence in Westerlund 1. Future multi-epoch surveys will be able to fully characterise this population.