Spectroscopic binaries in the open cluster Collinder 228

A radial velocity study of the members of the open cluster Collinder 228 is presented. For a total of 55 stars in the field of the cluster, an average of 10 spectra per star, is obtained and used to derive preliminary orbital elements for six new spectroscopic binaries. With the same observational material, projected axial rotation velocities are derived for stars later than O9 on the main sequence. A statistical discussion based on the above data indicated that the minimum proportion of binaries among the main sequence members of Collinder 228 in the spectral range O9-B3, is 31 percent while the average projected axial rotation of the cluster members with respect to the average rotation for field stars of the same types is 0.89 + or - 0.12. Several early-type SB2 systems of potential astrophysical interest are pointed out. 20 refs.