Magnitude-range brightness variations of overactive K giants.

Context. Decades-long, phase-resolved photometry of overactive spotted cool stars has revealed that their long-term peak-to-peak light variations can be as large as one magnitude. Such brightness variations are too large to be solely explained by rotational modulation and/or a cyclic, or pseudo-cyclic, waxing and waning of surface spots and faculae as we see in the Sun. Aims. We study three representative, overactive spotted K giants (ILHya, XXTri, and DMUMa) known to exhibit V-band light variations between 0.^m 65-1.^m 05. Our aim is to find the origin of their large brightness variation. Methods. We employ long-term phase-resolved multicolor photometry, mostly from automatic telescopes, covering 42 yr for ILHya, 28 yr for XXTri, and 34 yr for DMUMa. For one target, ILHya, we present a new Doppler image from NSO data taken in late 1996. Effective temperatures for our targets are determined from all well-sampled observing epochs and are based on a V - I_C color-index calibration. Results. The effective temperature change between the extrema of the rotational modulation for IL Hya and XX Tri is in the range 50-200 K. The bolometric flux during maximum of the rotational modulation, i.e., the least spotted states, varied by up to 39% in ILHya and up to 54% in XXTri over the course of our observations. We emphasize that for ILHya it is just about half of the total luminosity variation that can be explained by the photospheric temperature (spots/faculae) changes, while for XXTri it is even about one third. The long-term, 0.^m 6 V-band variation of DMUMa is more difficult to explain because little or no B - V color index change is observed on the same timescale. Placing the three stars with their light and color variations into H-R diagrams, we find that their overall luminosities are generally too low compared to predictions from current evolutionary tracks. Conclusions. A change in the stellar radius due to strong and variable magnetic fields during activity cycles likely plays a role in explaining the anomalous brightness and luminosity of our three targets. At least for ILHya, a radius change of about 9% is suggested from m_bol and T_eff, and is supported by independent v sin i measurements. [ABSTRACT FROM AUTHOR]