Upper panel: movement of the outer layers of a specific dynamic model atmosphere with dust formation occuring every second pulsation period (Appendix A). Colour-coded is the degree of condensation of carbon atoms into amorphous carbon dust particles. Lower panel: synthetic lightcurves for the broad-band filters V and K (filled circles), together with the corresponding photometry for which the dust opacities were neglected (open circles). Note that the K-lightcurves were shifted by 3^mag to better fit into one panel.