  The first substellar companion of possibly planetary mass around a normal
star, GQ Lup, has been directly imaged (Neuhaeuser et al., 2005). Besides the
unknown formation history, the mass of such an object is a criterion to decide
about its true nature. We aim to determine the physical properties of the GQ
Lup companion - effective temperate and surface gravity, and thus its mass
independently from evolutionary models. We use the adaptive optics
integral-field spectrograph SINFONI at the VLT for near-infrared spectroscopy
from 1.1 to 2.5 um with a resolution of R = 2500--4000. We compare these
spectra with synthetic atmospheric models (GAIA v2.0 cond). From the complete
set of spectra we find a consistent effective temperature of Teff = 2600 +/-
100 K and surface gravity of log{g} = 3.7 +/- 0.5. Combined with a slightly
revised luminosity of log(L/L_(\sun)) = -2.25 +/- 0.24 for the companion, we
determine a radius of R = 3.50 (+1.50/-1.03) Jupiterradii and thus a mass of ~
25 Jupitermasses. Due to the large uncertainty of the surface gravity, the mass
could range from 4 to 155 Jupitermasses. By comparing the paramaters of the
companion of GQ Lup to the ones of 2MASS J05352184-0546085, published by
Stassun et al. (2006), we conclude that the companion to GQ Lup A has a mass
lower than 36 Jupitermasses.
