X-ray and optical observations of M55 and NGC 6366: evidence for primordial binaries C. G. Bassa, D. Pooley, F. Verbunt, L. Homer, S. F. Anderson and W. H. G. Lewin A&A, 488 3 (2008) 921-933 Published online: 01 July 2008 DOI: 10.1051/0004-6361:200809350