  We present the discovery of a brown-dwarf companion to the 130-400 Myr-old G8
V star HD 203030. Separated by 11.9" (487 AU in projection) from its host star,
HD 203030B has an estimated mass of 0.023 (+0.008;-0.011) solar masses. The
K-band spectral type of L7.5+/-0.5 places HD 203030B near the critical L/T
transition in brown dwarfs, which is characterized by the rapid disappearance
of dust in sub-stellar photospheres. From a comparative analysis with
well-characterized field L/T transition dwarfs, we find that, despite its young
age, HD 203030B has a bolometric luminosity similar to the >1 Gyr-old field
dwarfs. Adopting a radius from current models of sub-stellar evolution, we
hence obtain that the effective temperature of HD 203030B is only 1206
(+74;-116) K, markedly lower than the ~1440 K effective temperatures of field
L/T transition dwarfs. The temperature discrepancy can be resolved if either:
(1) the ages of field brown dwarfs have been over-estimated by a factor of
\~1.5, leading to under-estimated radii, or (2) the lower effective temperature
of HD 203030B is related to its young age, implying that the effective
temperature at the L/T transition is gravity-dependent.
