  We present a method which provides some information on the possible binary
nature on an appararently single star. The method compares the instantaneously
measured HIPPARCOS proper motion with the long-term averaged, ground-based
proper motion or with the proper motion derived from old ground-based positions
and the HIPPARCOS position. Good sources for such ground based data are the FK5
and the GC. If the proper-motion difference Delta-mu is statistically
significant with respect to its measuring error, the object is very probably a
double star. We call then the object a 'delta-mu binary'. If the proper-motion
difference is insignificant and if no other information on a binary nature of
the object is available, we call such a star a 'single-star candidate'. We
propose a quantitative test for the significance of the observed proper-motion
difference. The sensitivity of our method is high: For nearby stars at a
distance of 10 pc, the measuring accuracy of the proper-motion difference,
expressed as a velocity, is of the order of 50 m/s (basic FK5 stars) or 80 m/s
(GC stars). At 100 pc, the mean error of the two-dimensional difference is
still 0.5 km/s or 0.8 km/s. For the FK5 stars, we provide indications on the
probable period of the delta-mu binaries. If we adopt an orbital period and a
mass-luminosity relation, we can use the observed velocity difference to
estimate the separation and the magnitude difference between the two components
of the binary. The present paper concentrates mainly on the basic principles of
the method, but it provides also a few examples of delta-mu binaries and of
single-star candidates for illustration: Gamma UMa, Epsilon Eri, Iota Vir, 47
UMa, Delta Pav.
