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Aims. We present the results obtained from a long-term spectroscopic campaigndevoted to the multiplicity of O-type stars in the young open cluster NGC2244and in the Mon OB2 association. Methods. Our spectroscopic monitoring wasperformed over several years, allowing us to probe different time-scales. Foreach star, several spectral diagnostic tools are applied, in order to searchfor line shifts and profile variations. We also measure the projectedrotational velocity and revisit the spectral classification. Results. In oursample, several stars were previously considered as spectroscopic binaries,though only a few scattered observations were available. Our results now reveala more complex situation. Our study identifies two new spectroscopic binaries(HD46149 in NGC2244 and HD46573 in MonOB2). The first object is a long-perioddouble-lined spectroscopic binary, though the exact value of its period remainsuncertain and the second object is classified as an SB1 system with a period ofabout 10.67 days but the time series of our observations do not enable us toderive a unique orbital solution for this system. We also classify another staras variable in radial velocity (HD46150) and we detect line profile variationsin two rapid rotators (HD46056 and HD46485). Conclusions. This spectroscopicinvestigation places a firm lower limit (17%) on the binary fraction of O-starsin NGC2244 and reveals the lack of short-period O+OB systems in this cluster.In addition, a comparison of these new results with two other well-studiedclusters (NGC6231 and IC1805) puts forward possible hints of a relation betweenstellar density and binarity, which could provide constraints on the theoriesabout the formation and early evolution of hot stars.

Early-type stars in the young open cluster NGC 2244 and in the Mon OB2 association I. The multiplicity of O-type stars