Upper panel: wavelength-dispersed flux ratio values of the best-fitting models. Other panels: contributions by different sources in typical spectra of C-rich AGB stars for the MIDI wavelength range. The synthetic spectra are plotted with continuum-normalised fluxes F/F_c, for detailed descriptions see Aringer et al. (2009) or Nowotny et al. (2010). The middle panel illustrates the individual molecular absorption due to C₂H₂ and HCN with the help of a hydrostatic model atmosphere, for details we refer to Fig. 4 of Aringer et al. (2009). The lower panel shows the characteristic emission due to circumstellar SiC dust on the basis of a dynamic model atmosphere (e.g. Höfner et al. 2003; Nowotny et al. 2010), computed following the approach of Sacuto et al. (2011) to artificially impose SiC grains. This dust species was detected in the ISO spectrum of R For (Clément et al. 2003).