The residual spectrum after removing telluric and stellar lines at the companion position. Overplotted in red is the CO model from the top panel of Fig. 3, convolved to the CRIRES resolution, rotationally broadened, and Doppler shifted to match the measured vsini (5.3 km s^-1) and radial velocity (2 km s^-1) of GQ Lupi b. For illustration purposes, the model was also fitted to the residual companion spectrum with a vertical offset and a scaling factor. The companion spectrum is dominated by noise, but the individual CO lines are discernible.