WR 20a: A massive cornerstone binary system comprising two extreme early-type stars G. Rauw, M. De Becker, Y. Nazé, P. A. Crowther, E. Gosset, H. Sana, K. A. van der Hucht, J.-M. Vreux and P. M. Williams A&A, 420 2 (2004) L9-L13 Published online: 28 May 2004 DOI: 10.1051/0004-6361:20040150