Evidence for a co-moving sub-stellar companion of GQ Lup

We present a companion of the <= 2 Myr young classical T Tauri star GQ Lup in the Lupus star forming region at 140 +/- 50 pc from imaging, astrometry, and spectroscopy. With direct K-band imaging using VLT/NACO, we detected an object 6 mag fainter than GQ Lup located 0.7 '' west of it. Compared to images obtained 2 to 5 years earlier with Subaru/CIAO and HST/PC, this object shares the proper motion of GQ Lup by 5 and 7 sigma, respectively, hence it is a co-moving companion. Its K-L' color is consistent with a spectral type early to mid L. Our NACO K-band spectrum yields spectral type M9-L4 with H2O and CO absorption, consistent with the new GAIA-Dusty template spectrum for log g similar or equal to 2 to 3 and T-eff similar or equal to 2000 K with similar to 2 R-jup radius at similar to 140 pc, hence few Jupiter masses. Using the theoretical models from Wuchterl & Tscharnuter ( 2003), Burrows et al. ( 1997), and Bara. e et al. ( 2002), the mass lies between 1 and 42 Jupiter masses.