1998JAVSO..26..112P

There are two RV Tauri stars in the AAVSO photoelectric photometry program. AC Herculis is a so-called RVa star: it has no significant long-term variations in mean magnitude. U Monocerotis is an RVb star: it has conspicuous long-term variations in mean magnitude, on a time scale of about 2500 days. The light curves of AC Her and U Mon are used to illustrate the RVa and RVb classification. The cause of the RVb phenomenon is poorly understood; current theories will be reviewed briefly. Times of minimum of AC Her and U Mon have been determined from the light curves, and added to the database compiled by E. Zsoldos (Konkoly Observatory, Hungary). The long-term (O-C) diagrams have been derived. Although these are dominated by the effects of random cycle-to-cycle fluctuations in period, it is possible to begin to see the effects of stellar evolution. The majority of the stars have decreasing periods, as would be expected for the stars in the post-asymbiotic-giant branch phase of evolution.