  Binary stars are present in all stellar systems, yet their role is far from
being fully understood. We investigate the effect of unresolved binaries in the
derivation of open clusters' mass by star counts. We start from the luminosity
functions of five open clusters: IC 2714, NGC 1912, NGC 2099, NGC 6834 and NGC
7142. Luminosity functions are obtained via star counts extracted from the
2MASS database. The fraction of binaries is considered to be independent on
stellar magnitude. We take into account different assumptions for the binary
mass ratio distribution and assign binary masses using the so-called {\it
luminosity-limited pairing} method and Monte-Carlo simulations. We show that
cluster masses increase when binary stars are appropriately taken into account.
