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AC Her and U Mon: RV Tauri Stars in the AASVO Photoelectric Photometry Program
There are two RV Tauri stars in the AAVSO photoelectric photometryprogram. AC Herculis is a so-called RVa star: it has no significantlong-term variations in mean magnitude. U Monocerotis is an RVb star: ithas conspicuous long-term variations in mean magnitude, on a time scaleof about 2500 days. The light curves of AC Her and U Mon are used toillustrate the RVa and RVb classification. The cause of the RVbphenomenon is poorly understood; current theories will be reviewedbriefly. Times of minimum of AC Her and U Mon have been determined fromthe light curves, and added to the database compiled by E. Zsoldos(Konkoly Observatory, Hungary). The long-term (O-C) diagrams have beenderived. Although these are dominated by the effects of randomcycle-to-cycle fluctuations in period, it is possible to begin to seethe effects of stellar evolution. The majority of the stars havedecreasing periods, as would be expected for the stars in thepost-asymbiotic-giant branch phase of evolution.
| Some Unpublished Photometric Observations of AC Herculis
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| A photometric investigation of RV Tauri and yellow semiregular variables
Results are presented for DDO and UBV photoelectric photometry of 52 RVTau and semiregular variables and candidates. CN abundances, effectivetemperatures, surface gravities, absolute visual magnitudes, and massestimates are derived in the framework of the spectroscopic groupingsproposed by Preston et al. (1963). The photometry suggests a furtherdivision of Preston's group A, and a possible physical connection isindicated between this group and the semiregular variables. Thespectroscopic groups are shown to be well separated in the DDOcolor-color diagrams when mean colors are used for individual stars. Anupper limit of about 3 solar masses is determined for stars in eachgroup. A correlation between derived iron abundances and published IRexcesses obtained from flux measurements at 3.6 and 11.3 microns isfound which supports the contention that dust production incircumstellar shells increases with increasing metallicity.
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