Comparison of pre- and post-transit reference spectra. Top plot, top panel: Averaged, normalized pre-transit spectra (nos. 1,2,3) and post-transit spectra (nos. 20 − 23) around the He Iλ10833 lines during night 1. Dotted lines indicate the averaged spectra obtained without our telluric emission line correction. Top plot, bottom panel shows the ratio of the averaged post- and pre-transit spectra along with an estimate of the 95 % confidencerange resulting from noise (green shades). The bottom panels show the same for the Hα line region.