  We present the discovery of 2MASS J11263991-5003550 identified as part of an
ongoing survey to discover ultra-cool dwarfs in the Southern Galactic Plane,
using data from the 2MASS and SuperCOSMOS Sky Surveys. Strong FeH and H2O
absorption features in the near-infrared JH-band spectrum reveal
characteristics seen in both mid-L, and L-T transition type dwarfs. We suggest
these may be attributable to holes in the condensate cloud layers in the
atmosphere of a single sub-stellar object, but cannot at present completely
rule out the role of binarity as the possible cause. We also identify this
object as a blue L dwarf, and explore the similar observable characteristics of
these objects with those of the L-T transition. From this comparison we suggest
that the temperature (and thus spectral type) at which the condensate cloud
later begins to break-up/rain out, may be highly sensitive to small variations
in metallicity. However, the JH-band spectrum of this object does not resemble
that of the known L sub-dwarfs, and therefore extreme metal deficiency may not
in fact be responsible for the discordant features. We estimate a spectral type
of L9+/-1, and measure a large proper-motion of \mu_(tot)=1.65"+/-0.03" per
year. Also, a spectrophotometric distance of 8.2pc is estimated, possibly
making this object the nearest easily observable single L-T transition object
in the southern hemisphere.
