Period Changes in RV Tauri and SRd Variables

Volume 33 number 2 (2005)

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John R. Percy

Jaime Coffey

Department

Abstract

RV Tauri (RVT) stars and semiregular pulsating variable supergiants of spectral types F, G, and K (SRd variables in the General Catalogue of Variable Stars) are undergoing "blue loops" from the asymptotic giant branch (AGB) in the Hertzsprung-Russell diagram, or are in transition from the AGB to the white dwarf stage. They should therefore show period changes due to their evolution. We have studied five such stars--AG Aur, AV Cyg, SX Her, UZ Oph, and TX Per--using up to a century of data, including data from the AAVSO International Database. We show (O-C) diagrams for these stars, fit parabolae to these, and calculate the characteristic time scales of period change. These five stars also, however, show strong evidence of random cycle-to-cycle period fluctuations, such as have been found in other stars of these types. These complicate the interpretation of the (O-C) diagrams in terms of evolutionary period changes, but the time scales that we derive are not inconsistent with the expected evolutionary time scales.