A VLT/FLAMES survey for massive binaries in Westerlund 1 - I. First observations of luminous evolved starsB. W. Ritchie, J. S. Clark, I. Negueruela and P. A. CrowtherA&A, 507 3 (2009) 1585-1595DOI: https://doi.org/10.1051/0004-6361/200912686