  We present time-resolved J-band spectroscopy of the short period cataclysmic
variable SDSS J143317.78+101123.3. We detect absorption lines from the
sub-stellar donor star in this system, which contributes 38 +/- 5% to the
J-band light. From the relative strengths of the absorption lines in the
J-band, we estimate the spectral type of the donor star to be L2 +/- 1. These
data are the first spectroscopic detection of a confirmed sub-stellar donor in
a cataclysmic variable, and the spectral type is consistent with that expected
from semi-empirical evolutionary models.
  Using skew mapping, we have been able to derive an estimate for the radial
velocity of the donor of Kd = 520 +/- 60 km/s. This value is consistent with,
though much less precise than, predictions from mass determinations found via
photometric fitting of the eclipse light curves.
