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 that allows investigation of the dependence of synthetic velocity dispersion on a number of crucial quantities characterizing the binary content: binary fraction and the

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... ributions of their mass ratio, eccentricity, and semimajor axis. As an application, we evaluate the impact that binary stars have on the estimation of the dynamical mass of dwarf spheroidal and ultrafaint 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.