Illustration of the effect of dust formation on the photometric appearance of the star, demonstrated with the help of model S. Upper panel: movement of atmospheric layers of the model over about two light cycles, colour-coded is the degree of condensation of carbon into dust grains. Middle panel: optical depths of the dust envelope as a function of time, calculated for the central wavelengths of the filters V and K following Eq. (1). Lower panel: resulting synthetic lightcurves for the same filters (filled circles), together with the corresponding photometry for which the dust opacities were not included (open circles). Note that the K-lightcurves were shifted by 5^mag to fit into the panel (cf. Fig. 7 in Paper II).