Wavelength calibration of the AMBER observations. A telluric spectrum from the Kitt Peak Observatory (KPO) is shown in comparison with AMBER spectra of Rigel and 31 Ori around the Brγ line. The dashed vertical lines indicate the positions of the two telluric lines used as wavelength reference. The telluric line doublet at 2.1670 μm seen in the KPO spectrum does not feature in the 31 Ori spectrum. Therefore, the Brγ line profile of Rigel is bona fide undisturbed by telluric lines.