Light-curve and period changes of AB-Andromedae
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New photoelectric observations of AB And obtained between 1989 and 1992 have been used, together with all available light curves from the literature, to study the light-curve variation of the system and its probable connection with the orbital period variation. A long-term brightness variation in the light levels is detected. The maximum brightness occurred in 1970 (+/- 3 yr). A period study based on all available times of minimum light (of which 22 are newly determined) reveals that the photometric period of the system oscillates around a mean value (P0 almost-equal-to 0.331 890 d) with an oscillation period of almost-equal-to 88.0 +/- 0.2 yr and a half-amplitude of 0.000 003 76 d. Such a variation can be caused by either (i) a period modulation due to the magnetic activity cycle of the active primary component, or (ii) a light-traveltime effect due to a third body in the system. It is shown that the third body, if it exists, can only be a white dwarf; this can be checked by ultraviolet spectroscopy. The rms light variation of the system predicted by Applegate's theory of the magnetic activity modulation of the orbital period is found to be comparable with the observed amplitude of long-term brightness variations of the system. However, the brightness variation is found to be 90-degrees out of phase with the O - C curve. Such a phase shift can be explained in terms of the damping effect of the convective zone. The theory predicts a subsurface magnetic field of 7.4 kG for the primary component of AB And.