Normalized GRAVITY spectrum at the position of the He I 2s-2p and Brγ lines. The calibrated differential visibilities (middle) and phases (bottom) of one of the GRAVITY data sets (MJD: 57903.9840) are also shown. In the middle and bottom panels, different colors correspond to each one of the baselines in the data set (see label on the plots). The dashed-red vertical lines indicate the reference wavelength of the emission feature, and the dashed-horizontal lines show the reference value of the continuum baseline.