Detection of a Stellar Stream Behind Open Cluster NGC 188: Another Part
  of the Monoceros Stream
  We present results from a WIYN/OPTIC photometric and astrometric survey of
the field of the open cluster NGC 188 ((l,b) = (122.8\arcdeg, 22.5\arcdeg)). We
combine these results with the proper-motion and photometry catalog of Platais
et al. and demonstrate the existence of a stellar overdensity in the background
of NGC 188. The theoretical isochrone fits to the color-magnitude diagram of
the overdensity are consistent with an age between 6 and 10 Gyr and an
intermediately metal poor population ([Fe/H] = -0.5 to -1.0). The distance to
the overdensity is estimated to be between 10.0 and 12.6 kpc. The
proper-motions indicate that the stellar population of the overdensity is
kinematically cold.
  The distance estimate and the absolute proper motion of the overdensity agree
reasonably well with the predictions of the Pe\~{n}arrubia et al. model of the
formation of the Monoceros stream. Orbits for this material constructed with
plausible radial-velocity values, indicate that dynamically, this material is
unlikely to belong to the thick disk. Taken together, this evidence suggests
that the newly-found overdensity is part of the Monoceros stream.
