Fits of different models to the visibility measurements for wavelength bin 6 (9.07 μm). The 75 visibility data, taken over all pulsation phases and cycles, are plotted versus spatial frequency. Left: the measurements color-coded by visual light phase. The fit of a circular FDD gives for this wavelength a diameter of 79.2 mas and a relative flux contribution of 0.85. In addition, it can be clearly seen that a UD does not fit the data as well as an FDD. Right: the measurements color-coded by position angle. The elliptical FDD model fit results along the minor (76 mas) and major (85 mas) axis are included as solid lines. The fit to a given measurement is represented as a small open circle and is connected by a line with the corresponding observational point. The relative flux contribution is determined to be 0.86.