RV Tauri stars

Christy found that a nonlinear pulsation model intended to represent W Vir exhibited RV Tauri-like light variations, i.e. alternately deep and shallow minima. The linear periods of the fundamental and 1st-overtone modes were found to satisfy roughly P1/Po=2/3 and Christy proposed that this resonance was the cause of RV Tauri light variations. Since that time the same coincidence of RV Tauri-like light curve and resonance have been found by other authors. Note that in this explanation of RV Tauri behaviour the time between successive light minima is Po. Using linear, adiabatic pulsation models Takeuti and Petersen investigated a similar hypothesis in which the time between successive minima was taken to be 1/2xPo and the RV Tauri behaviour was the result of a two-mode, e.g. P1/Po=1/2, or a three-mode resonance. They found a negative result. However, if, as now seems likely, the RV Tauri stars are very closely related to, but rather brighter than, the W Vir stars nonadiabatic effects should be very important. It has been shown that a high L/M when nonadiabatic effects are important P1/Po is much greater than the value obtained from adiabatic calculations