Photoelectric Photometry Newsletter

RS CVn Stars in the AAVSO PEP Program

The RS CVn stars have been defined by Hall (1976, in Multiple Period Variable Stars, ed. W.S. Fitch, D. Reidel, p. 287) as "binaries with orbital periods between 1 and 14 days, with the hotter component F-G IV-V and with strong H and K emission seen in the spectrum outside eclipse". In fact, there are similar stars with orbital periods less than a day ("the short-period group") and greater than 14 days ("the long-period group"). It is probably appropriate to lump all these stars together and call them "stars showing the RS CVn phenomenon".

Some of these stars are eclipsing variable stars, but they all show additional out-of-eclipse variability which is due to large starspots (or starspot regions) which rotate into and out of the field of view. The period of variability is the period of rotation of the star. The amplitude of the rotational variability is variable, and the size of the starspot (region) changes. The relative phase of the rotational variability may vary as the starspot (region) migrates around the star.

These stars began to attract wide interest in the 1970's, not just because of their photometric variability, but because of exotic properties which are reminiscent of solar activity, but on a much greater scale - radio emission and flaring, thermal X-ray emission indicating temperatures of 10,000,000 K, and strong and variable emission lines. This extreme stellar "activity" is caused by the relatively rapid rotation of the star, which results (in most cases) from tidal interaction between the star and its companion.

Because of this interest, three RS CVn stars were placed on the AAVSO Photoelectric Photometry Program in the early 1980's. These stars are relatively easy to observe: the colours are moderate; it is easier to find comparison stars than for the extremely red stars on the program; and the stars are a subject of interest. The AAVSO PEP observations of these three stars are presented here for the first time.

HK Lac

The data were divided into two-to-three-year segments. The amplitude of the V phase curve is about 0.20, 0.25, and 0.10 in the three segments; the variation is due to an increase (large amplitude) and decrease (small amplitude) in the size of the starspot. There is also a slight variation of the level of maximum magnitude; in the phase curve for the second segment, when the star is heavily spotted, the maximum magnitude is slightly fainter. The phase of the maximum of the phase curve also changes as the spot migrates in longitude (though it is difficult to keep track of this in a star which does not show eclipses).

SZ Psc

Four V-magnitude phase curves for SZ Psc were calculated; the data were divided into approximately three-year intervals. The eclipses were visible at the same phase. The amplitude and phase of the non-eclipse variations changed, due to the spots. (The data on this star are rather sparse.)

We will be using the phase of the eclipse to investigate the period change in the eclipsing variable.

Lambda And

Four V -magnitude phase curves for Lambda And were calculated; they correspond to approximately one-year intervals when the data were most numerous. There was a conspicuous change in the amplitude.

Further Work

Acknowledgements. We thank the observers who have gathered the data for these stars over the years, and we than the AAVSO HQ staff for their support. We especially thank Howard J. Landis, the Archivist for the AAVSO PEP Program, for his help and support.