a) Normalized K-band spectrum (with a binning over 2 spectral channels) obtained with GRAVITY. The Brγand He i lines as well as the expected position of the corresponding jet lines (jet1/jet2 for the approaching/receding jet) are indicated by vertical lines. We have also reported the expected position of the Brδ_jet2 line from the receding jet. The solid blue/red line corresponds to the emission of the approaching/receding jet components. b) Visibility amplitudes and c) phases on the UT1-UT3 baseline (the visibility amplitudes and phases of all the baselines as well as the uv-plane at the time of the observation are shown in the Appendix, in Fig. B.1). For the phase, we follow the sign convention of Pauls et al. (2005), such that negative phases point to the baseline direction. The solid black line corresponds to the best fit model of the jet lines (see Sect. 3.2.2).