  Binary stars are recognized to be important in driving the dynamical
evolution of stellar systems and also in determining some of their
observational features. In this study, we explore the role that binary stars
have in modulating the estimates of the velocity dispersion of stellar systems.
To this aim, we developed a tool which allows to investigate the dependence of
synthetic velocity dispersion on a number of crucial quantities characterizing
the binary content: binary fraction and the distributions of their mass ratio,
eccentricity and semi-major axis. As an application, we evaluate the impact
that binary stars have on the estimation of the dynamical mass of dwarf
spheroidal and ultra-faint dwarf galaxies, finding that it can be particularly
relevant, especially for low mass and low density systems. These results bear
profound implications for the interpretation of the measured velocity
dispersion in such systems, since it weakens or relieves the claim for the need
of large amounts of dark matter.
