Semi-regular and alternating pulsations in hydrodynamical models for low-mass giants

On the basis of hydrodynamical simulations the semi-regular and alternating pulsations of the W Vir and RV Tau stars are studied including non-steady radiative transfer effects in gray sphericalsymmetric atmospheres. It is shown that the transition from regular to semi-regular oscillations occurs at periodsP13–15 days due to pulsation energy imbalance provoqued by strong radiating shocks. The differences between the periods and amplitudes of W Vir and RV Tau variables are explained as a result of this imbalance. It is suggested that the semi-regular alternating pulsations of the RV Tau stars appear following a period-doubling phenomenon, which can be also understood in terms of the pulsation energy variations between odd and even fundamental cycles. The pulsational characteristics of the period-doubled models are in general accordance with those observed in the RV Tau stars. On the basis of numberical results, a theoretical estimation of the upper limit for the luminosity of the RV Tau stars is derived asL10³ L .