2001A&A...379..878E

The nature of the long-term orbital period variation and the light level variation of RT And was studied based on all available photometric observations. The (O-C) diagram formed by all available times of eclipse minima could be either of a quasi-sinusoidal form superimposed on a parabolic form, or a long period sinusoidal form. The quasi-sinusoidal form with an amplitude of ∼0.002 days and a period of ∼65 years could be due to a cyclic magnetic activity modulation on the orbital period of the system. Applegate's theory (Applegate, 1992ApJ...385..621A), in this case, predicts an ∼3kG mean subsurface magnetic field for the primary component of the system. The parabolic form corresponds to a secular period decrease of about 3x10^–9m_☉/yr caused by mass loss from the system, and the long-period sinusoidal form with an amplitude of ∼0.02 days and a period of ∼105 years corresponds to a secular period change caused by the light-time effect due to an unseen third star in the system. It was estimated that the unseen third star should be a low mass (lower than 0.6m_☉) and faint (∼5mag fainter than the system RT And) star, whose detection is almost impossible. Future data will add information on the nature of the secular (O-C) variation. Secular light level variation of the system was found to be irregular and no correlation was detected with the orbital period variation.