Pulsation model of a RSG with M = 15 M_⊙, L = 126 000 L_⊙, T_eff ~ 3600 K. Bottom panel: radius variation of selected mass zones in a pulsating supergiant model with M = 15 M_⊙ and L = 126 000 L_⊙ (black curves). The red curve is the position of the photosphere (defined as the layer where the Rosseland optical depth equals 2/3). Middle panel: the velocity at the photosphere. Top panel: the visual light curve of the model, where the bolometric correction is obtained from the tables in Houdashelt et al. (2000a,b).