The 18 VLTI/AMBER φ_diff measured on Fomalhaut around Brγ at 7 different observing times (format YYYY-MM-DDTHH MM SS) and, for each time, three different projected baselines and baseline position angles, as indicated in the plots. The smooth curves superposed to the observations are the best-fit φ_diff obtained without a differential rotation (Domiciano de Souza et al. 2003), gravity-darkened Roche model, as described in Sect. 3. All the observed φ_diff points are shown here, knowing that the fit has been performed using all the wavelength points.