AMBER observations of MWC 297 with spectral resolution of 12 000: (from top to bottom) wavelength dependence of flux, visibilities, wavelength-differential phases (for better visibility, the differential phases of the shortest and longest baselines are shifted by +40 and  − 40°, respectively), and closure phase observed at projected baselines of 14.0, 28.0, and 42.1 m along the position angle (PA) of 68.0° on the sky. The wavelength scale at the bottom is the directly observed one, i.e. without heliocentric or LSR correction (see text). However, the radial velocity scale at the top, gives spectrum, visibilities, and phases as a function of the Brγ Doppler shift in the Local Standard of Rest (LSR) frame. The dashed vertical line indicates the centroid Brγ vacuum wavelength (2.1661 μm) in the LSR frame.