  We have re-analyzed archival HST and Subaru data of the recently discovered
planetary mass companion (PMC) GQ Lup B. With these we produce the first R and
I band photometry of the companion and fit a radius and effective temperature
using detailed model atmospheres. We find an effective temperature of 2335 +/-
100K, and a radius of 0.38 +/- 0.05 Rsol and luminosity of log(L/Lsol) = -2.42
+/- 0.07 (at 140pc). Since we fit wavelengths that span most of the emitted
radiation from GQ Lup this luminosity estimate is robust, with uncertainty
dominated by the distance uncertainty (+/- 50 pc). The radius obtained for
140pc (0.38Rsol) is significantly larger than the one originally derived and
larger than model predictions. The mass of the object is much more
model-dependent than the radiative properties, but for the GAIA dusty models we
find a mass between 10-20 MJup, in the range of the brown dwarf and PMC
deuterium burning boundary. Assuming a distance of 140pc, observations fit to
1sigma the Baraffe evolution model for a ~15 MJup brown dwarf. Additionally,
the F606W photometric band is significantly overluminous compared to model
predictions and other brown dwarfs. Such overluminosity could be explained by a
bright Halpha emission from chromospheric activity, interaction with another
undetected companion, or accretion. Assuming that GQ Lup B has a bright Halpha
emission line, its Halpha emission strength is 10^(-1.71 +/- 0.10) Lbol,
significantly larger than field late-type dwarfs. GQ Lup B might be strongly
accreting and still be in its formation phase.
