How do binaries affect the derived dynamical mass of a star cluster?

The dynamical mass of a star cluster can be derived from the virial theorem, using the measured half-mass radius and line-of-sight velocity dispersion of the cluster. However, this dynamical mass may be a significant overestimation of the cluster mass if the contribution of the binary orbital motion is not taken into account. Here, we describe the mass overestimation as a function of cluster properties and binary population properties, and briefly touch on the issue of selection effects. We find that for clusters with a measured velocity dispersion of sigma (los)a parts per thousand(3)10 km s(-1) the presence of binaries does not affect the dynamical mass significantly. For clusters with sigma (los)a parts per thousand(2)1 km s(-1) (i.e., low-density clusters), the contribution of binaries to sigma (los) is significant, and may result in a major dynamical mass overestimation. The presence of binaries may introduce a downward shift of Delta log (L (V) /M (dyn))=0.05-0.4 (in solar units) in the log (L (V) /M (dyn)) versus age diagram.