the Annual Meeting of the AAVSO, October 25, 1997
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.