  We report the discovery of a companion to the high proper motion star LHS
1589, a nearby high-velocity, low-mass subdwarf. The companion (LHS 1589B) is
located 0.224+/-0.004" to the southwest of the primary (LHS 1589A), and is 0.5
magnitude fainter than the primary in the K_s band. The pair was resolved with
the IRCAL infrared camera at Lick Observatory, operating with the Laser Guide
Star Adaptive Optics system. A low-resolution spectrum of the unresolved pair
obtained at the MDM observatory shows the source to be consistent with a cool
subdwarf of spectral subtype sdK7.5. A photometric distance estimate places the
metal-poor system at a distance d=81+/-18 parsecs from the Sun. We also measure
a radial velocity V_rad=67+/-8 km/s which, together with the proper motion and
estimated distance, suggests that the pair is roaming the inner Galactic halo
on a highly eccentric orbit. With a projected orbital separation s=18.1+/-4.8
AU, and a crude estimate of the system's total mass, we estimate the orbital
period of the system to be in the range 75 yr < P < 500 yr. This suggests that
the dynamical mass of the system could be derived astrometrically, after
monitoring the orbital motion over a decade or so. The LHS 1589AB system could
thus provide a much needed constraint to the mass-luminosity relationship of
metal-poor, low-mass stars.
