Photometry of yellow semiregular variables : AC Herculis, R Sagittae, and V Vulpeculae.

Three RV Tauri variables - AC Her, R Sge, and V Vul - have been observed photoelectrically since 1984. Their current periods are 75.48 d for AC Her, 69.74 d for R Sge, and 75.35 d for V Vul. The light curves of AC Her and R Sge are quite similar and show the usual RV Tauri-like form. The secondary minima of V Vul, however, is very shallow. Period analysis revealed the presence of the fundamental period and its harmonics (two or three). All of the three stars show cyclic period changes, the cause of which is still unknown. The expected long-term period change is not detected in R Sge and V Vul, but this is probably the result of the few observed cycles in the O-C diagram. Frequency clustering around one or two principal frequencies is found in all three stars. Possible causes for this phenomenon, such as amplitude or period changes, are suggested. The question of the pulsation period of RV Tauri stars is also discussed, the formal period being the preferred choice. Since all theoretical models fail to reproduce the periods of RV Tauri variables, there is no possibility to compare the observed properties of these stars to model predictions. Some discrepancies in the derived properties of AC Her are pointed out, too.