Wavelength calibration of the AMBER data. Top: raw AMBER spectrum of MWC 297 (solid blue line) compared to a telluric spectrum from the Kitt Peak Observatory. The green line shows the original Kitt Peak spectrum with a spectral resolution of 60 000, while the red line displays the telluric spectrum that has been convolved to the same spectral resolution as the AMBER data (R = 12   000). For clarity, the red curve has been shifted down by 0.4. As the figure reveals, after the wavelength calibration, the AMBER raw spectrum of MWC 297 matches several telluric features. Bottom: same as top panel, but just for the wavelength region around the Brγ line.