1999A&A...349..546T

The RS CVn type eclipsing binary MM Herculis was observed using B, V, and R filters in 1997 and the light and colour curves were obtained. The long-term (1976-1997) variations in the brightness and colour of the system were revealed by examining 12 light and colour curves. The reason for the variations in the B-V colour curves may be the photospheric bright facular structures that surround starspots. The system is bluest when at its faintest. Migration periods of 5.8 and 5.9 years have been found for the two spots or spot groups, which are located on the cooler component. The spots that are located within the longitude difference of 180° seem to approach each other with a period of six years. In this case the amplitudes of the light curves increase, the system's mean brightness decreases and its mean colour becomes bluer.