  The comparison of the proper motions constructed from positions spanning a
short (Hipparcos) or long time (Tycho-2) makes it possible to uncover binaries
with periods of the order of or somewhat larger than the short time span, since
the unrecognised orbital motion will then add to the proper motion. A list of
candidate proper motion binaries is constructed from a chi^2 test evaluating
the statistical significance of the difference between the Tycho-2 and
Hipparcos proper motions for 103134 stars in common between the two catalogues
(excluding components of visual systems). The paper focuses on the evaluation
of the detection efficiency of proper-motion binaries, using different kinds of
control data (mostly radial velocities). The detection rate for entries from
the Ninth Catalogue of Spectroscopic Binary Orbits (SB9) is evaluated, as well
as for stars like barium stars, which are known to be all binaries. Proper
motion binaries are efficiently detected for systems with parallaxes in excess
of ca. 20 mas, and periods in the range 1000 - 30000 d. The shortest periods in
this range (1000 - 2000 d) may appear only as DMSA/G binaries (accelerated
proper motion in the Hipparcos Double and Multiple System Annex). Proper motion
binaries detected among SB9 systems having periods shorter than about 400 d
hint at triple systems. A list of 19 candidate triple systems is provided.
Binaries suspected of having low-mass (brown-dwarf-like) companions are listed
as well. Among the 37 barium stars with parallaxes larger than 5 mas, only 7
exhibit no evidence for duplicity whatsoever (be it spectroscopic or
astrometric). Finally, the fraction of proper-motion binaries shows no
significant variation among the various (regular) spectral classes, when due
account is taken for the detection biases.
