Top: AMBER instrumental fringing effect on the differential phases as a function of wavelength. Left: Achernar’s differential phases showing high-frequency beating (black line), and the same filtered out (red line). Right: same as for Altair. Bottom: left: Achernar’s dynamical differential phases before removing the fringing effect, for the observing night of 2009 Nov. 01, for the E0-H0 baseline. Center: after bias removal, the rotation effect around the Brackett γ line becomes more visible as a dark vertical strip next to a bright vertical strip. Right: by projecting the differential phases as a function of wavelength, one gets the typical s-shaped effect as a function of wavelength.