Many young extra-galactic clusters have a measured velocity dispersion that is too high for the mass derived from their age and total luminosity , which has led to the suggestion that they are not in virial equilibrium .
Most of these clusters are confined to a narrow age range centred around 10 Myr because of observational constraints .
At this age the cluster light is dominated by luminous evolved stars , such as red supergiants , with initial masses of \sim 13 -22 \textup { M } _ { \odot } for which ( primordial ) binarity is high .
In this study we investigate to what extent the observed excess velocity dispersion is the result of the orbital motions of binaries .
We demonstrate that estimates for the dynamical mass of young star clusters , derived from the observed velocity dispersion , exceed the photometric mass by up-to a factor of 10 and are consistent with a constant offset in the square of the velocity dispersion .
This can be reproduced by models of virialised star clusters hosting a massive star population of which \sim 25 % is in binaries , with typical mass ratios of \sim 0.6 and periods of \sim 1000 days .
We conclude that binaries play a pivotal role in deriving the dynamical masses of young ( \sim 10 Myr ) moderately massive and compact ( \lesssim 10 ^ { 5 } \textup { M } _ { \odot } ; \gtrsim 1 pc ) star clusters .