  We explored the regions within a radius of 25 arcsec around 473 nearby,
low-metallicity G- to M-type stars using (VR)I optical filters and
small-aperture telescopes. About 10% of the sample was searched up to angular
separations of 90 arcsec. We applied photometric and astrometric techniques to
detect true physical companions to the targets. The great majority of the
sample stars was drawn from the Carney-Latham surveys; their metallicities
range from roughly solar to [Fe/H]=-3.5 dex. Our I-band photometric survey
detected objects that are between 0 and 5 mag fainter (completeness) than the
target stars; the maximum dynamical range of our exploration is 9 mag. We also
investigated the literature and inspected images from the Digitized Sky Surveys
to complete our search. By combining photometric and proper motion
measurements, we retrieved 29 previously known companions, and identified 13
new proper motion companions. Near-infrared 2MASS photometry is provided for
the great majority of them. Low-resolution optical spectroscopy (386-1000 nm)
was obtained for eight of the new companion stars. These spectroscopic data
confirm them as cool, late-type, metal-depleted dwarfs, with spectral classes
from esdK7 to sdM3. After comparison with low-metallicity evolutionary models,
we estimate the masses of the proper motion companion stars to be in the range
0.5-0.1 Msol. They are orbiting their primary stars at projected separations
between ~32 and ~57000 AU. These orbital sizes are very similar to those of
solar-metallicity stars of the same spectral types. Our results indicate that
about 15% of the metal-poor stars have stellar companions at large orbits,
which is in agreement with the binary fraction observed among main sequence G-
to M-type stars and T Tauri stars.

Astrophysics