Early-type stars in the young open cluster NGC 2244 and in the Monoceros OB2 association - I. The multiplicity of O-type starsL. Mahy, Y. Nazé, G. Rauw, E. Gosset, M. De Becker, H. Sana and P. EenensA&A, 502 3 (2009) 937-950DOI: https://doi.org/10.1051/0004-6361/200911662