A spectroscopic and proper motion search of Sloan digital Sky Survey:
  red subdwarfs in binary systems
  Red subdwarfs in binary systems are crucial for both model calibration and
spectral classification. We search for red subdwarfs in binary systems from a
sample of high proper motion objects with Sloan digital Sky Survey
spectroscopy. We present here discoveries from this search, as well as
highlight several additional objects of interest. We find 30 red subdwarfs in
wide binary systems including: two with spectral type of esdM5.5, 6 companions
to white dwarfs and 3 carbon enhanced red subdwarfs with normal red subdwarf
companions. 15 red subdwarfs in our sample are partially resolved close binary
systems. With this binary sample, we estimate the low limit of the red subdwarf
binary fraction of ~ 10%. We find that the binary fraction goes down with
decreasing masses and metallicities of red subdwarfs. A spectroscopic esdK7
subdwarf + white dwarf binary candidate is also reported. 30 new M subdwarfs
have spectral type of >= M6 in our sample. We also derive relationships between
spectral types and absolute magnitudes in the optical and near-infrared for M
and L subdwarfs, and we present an M subdwarf sample with measured U, V, W
space velocities.
