VLTI/MIDI interferometry at 8–13 μm of RR Aql for the example of epoch A (stellar phase 0.58) and epoch I (stellar phase 2.75) . The panels show a) the flux; b) the visibility amplitude; c) the corresponding UD diameter; d) the corresponding Gaussian FWHM diameter as a function of wavelength. The gray shading indicates the wavelength region around 9.5 μm that is affected by atmospheric absorption. Panel e) shows the visibility amplitude as a function of spatial frequency for three averaged bandpasses of 8–9 μm, 10–11 μm, and 12–13 μm. The crosses with error bars denote the measured values. The solid lines indicate our best-fitting model, as described in Sect. 5. It consists of a combination of a dust-free dynamic model atmosphere representing the central star and a radiative transfer model representing the surrounding dust shell. The contributions of the stellar and dust components alone are indicated by the dotted and the dashed lines, respectively.