On the Nature of Regular Pulsation in Two LBV Stars of NGC 300

Two A-type supergiants in NGC 300 exhibit periodic light variations. The measured periods are 72.5 and 96.1 days. We show that the variability may be explained by the excitation of strange mode driven by the opacity mechanism acting in the layer of the iron opacity bump. The explanation requires that the two stars are significantly undermassive. In sequences of stellar models with decreasing luminosity-to-mass ratio, the unstable strange modes represent continuations of strictly aperiodic thermal modes.