Ultra-cool dwarfs: new discoveries, proper motions, and improved
  spectral typing from SDSS and 2MASS photometric colors
  Aims. We try to identify ultra-cool dwarfs from the seventh Data Release of
the Sloan Digital Sky Survey (SDSS DR7) with SDSS i-z and r-z colors. We also
obtain proper motion data from SDSS, 2MASS, and UKIDSS and improve spectral
typing from SDSS and 2MASS photometric colors. Methods. We selected ultra-cool
dwarf candidates from the SDSS DR7 with new photometric selection criteria,
which are based on a parameterization study of known L and T dwarfs. The
objects are then cross-identified with the Two Micron All Sky Survey and the
Fourth Data Release of the UKIRT Infrared Deep Sky Survey (UKIDSS DR4). We
derive proper motion constraints by combining SDSS, 2MASS, and UKIDSS
positional information. In this way we are able to assess, to some extent, the
credence of our sample using a multi epoch approach, which complements
spectroscopic confirmation. Some of the proper motions are affected by short
baselines, but, as a general tool, this method offers great potential to
confirm faint L dwarfs as UKIDSS coverage increases. In addition we derive
updated color-spectral type relations for L and T dwarfs with SDSS and 2MASS
magnitudes. Results. We present 59 new nearby M and L dwarfs selected from the
imaging catalog of the SDSS DR7, including proper motions and spectral types
calculated from the updated color-spectral type relations. and obtain proper
motions from SDSS, 2MASS, and UKIDSS for all of our objects.
