X-ray and optical observations of M55 and NGC 6366: evidence for primordial binariesC. G. Bassa, D. Pooley, F. Verbunt, L. Homer, S. F. Anderson and W. H. G. LewinA&A, 488 3 (2008) 921-933DOI: https://doi.org/10.1051/0004-6361:200809350