  We analyze 9 open clusters with ages in the range 70 Myr to 3.2 Gyr using
UCAC2 proper motion data and 2MASS photometry. For each cluster we consider the
projected velocity distributions in the core and off-core regions separately.
In the projected velocity distribution of all sample clusters we find a
well-defined low-velocity peak, as well as an excess in the number of stars at
larger velocities. The low-velocity peak is accounted for by the random motion
of the single stars, while the high-velocity excess can be attributed to the
large velocity changes produced by a significant fraction of unresolved
binaries in a cluster. We derive kinematic parameters of the single-star
distribution, in particular the projected velocity dispersion. The relatively
large velocity dispersions derived in this work may reflect the non-virialized
state of the clusters. Based on the relative number of high-velocity (binary)
and single stars, we inferred for the sample clusters unresolved binary
fractions in the range $15%-54%$, for both core and off-core regions. The
present results suggest that care must be taken when applying proper-motion
filters to sort out members, especially binaries in a star cluster. This paper
shows that proper motions turn out to be a useful tool for identifying
high-velocity stars as unresolved binary cluster members, and as a consequence,
map and quantify the binary component in colour-magnitude diagrams.
