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 $140 \pm 50$ 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 $0.7^{\prime \prime}$ 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 $\log~g \simeq 2$ to 3 and $T_{\rm eff} \simeq 2000$ K with ~2 Rjup radius at ∼140 pc, hence few Jupiter masses. Using the theoretical models from Wuchterl & Tscharnuter (2003), Burrows et al. (1997), and Baraffe et al. (2002), the mass lies between 1 and 42 Jupiter masses.