Simulation 1. Synthetic flux (top) and visibility amplitude (bottom) in the wavelength range 8–13 μm. The solid lines represent the global models, the dashed-dotted lines denote the unattenuated stellar contribution i.e. V_star in Eq. (2), the dotted lines denote the attenuated stellar contribution, and the dashed lines denote the dust shell contributions. This simulation compares two models consisting of the same dust shell parameters but different model atmospheres, the post-maximum model atmosphere M21n (Φ_model = 0.10), and the minimum model atmosphere M23n (Φ_model = 0.30). The photospheric angular diameter is assumed to be larger at post-maximum phase than at minimum phase (8.5 mas compared to 7 mas). The projected baseline length is 60 m. For the exact model parameters, see Table 4. They describe variations around our best-fitting model for RR Aql.