Photoelectric Photometry Newsletter

RV TAURI Stars in the AAVSO Photoelectric Photometry Program

RV Tauri stars, according to the General Catalogue of Variable Stars, are: "pulsating supergiants having spectral types F to G at maximum light and K to M at minimum light. The light curves are characterized by the presence of double waves with alternating primary (deep) and secondary (shallow) minima which can vary in depth so that the primary minima may become secondary ones, and vice versa. The complete light amplitude may reach 3 to 4 magnitudes in V. The periods between two adjacent primary minima (called "double" or "formal" periods) lie in the range 30 to 150 days. Two subtypes may be isolated: RVa - variables of RV Tauri type which do not vary in mean magnitude; RVb - variables of RV Tauri type which periodically in mean magnitude with periods of 600 to 1500 days (or more), and amplitudes up to 2 magnitudes in V."

The cause of the alternating deep and shallow minima is generally agreed on; it is due to the presence of two pulsation periods in the ratio 2:1. The cause of the long-term variations is not known for sure.

In the AAVSO Photoelectric Photometry Program, there are two RV Tauri stars: AC Her, an RVa star, and U Mon, an RVb star. Below, we show the light curves of these stars on both the long-term and the short term.

Fig. 1. The long-term light curve of AC Her. The vertical scatter is due to the pulsation, which has a double period of 75.41 days.

Fig. 2. The phase diagram of AC Her, using the double period of 75.41 days. The low scatter indicates that there is no long-term variation, or other irregularity.

Fig. 3. The long-term light curve of U Mon, an RVb star. Note the variation in mean magnitude, on a time scale of 2500 days. The vertical scatter is due to the pulsation, which has a double period of 92.23 days.

Fig. 4. The short-term light curve of U Mon, showing the alternating deep and shallow minima. The double period is 92.23 days.

We have used these light curves to determine precise times of minimum brightness of these two stars, and we are using these, along with many decades of data provided by Endre Zsoldos (Konkoly Observatory, Hungary) to study the period changes and evolution of RV Tauri stars.