LIGHT-CURVE AND PERIOD CHANGES OF AB ANDROMEDAE

New photoelectric observations of AB And obtained between 1989 and 199 2 have been used, together with all available light curves from the li terature, to study the light-curve variation of the system and its pro bable connection with the orbital period variation. A long-term bright ness variation in the light levels is detected. The maximum brightness occurred in 1970 (+/- 3 yr). A period study based on all available ti mes of minimum light (of which 22 are newly determined) reveals that t he photometric period of the system oscillates around a mean value (P0 almost-equal-to 0.331 890 d) with an oscillation period of almost-equ al-to 88.0 +/- 0.2 yr and a half-amplitude of 0.000 003 76 d. Such a v ariation can be caused by either (i) a period modulation due to the ma gnetic 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 sy stem predicted by Applegate's theory of the magnetic activity modulati on of the orbital period is found to be comparable with the observed a mplitude of long-term brightness variations of the system. However, th e 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 damp ing effect of the convective zone. The theory predicts a subsurface ma gnetic field of 7.4 kG for the primary component of AB And.