The Light and Period Variations of the Eclipsing Binary AA Ursae Majoris

Abstract

ABSTRACTWe present new multiband CCD photometry for AA UMa made on eight nights between 2009 January and March; the R light curves are the first ever compiled. Historical light curves, as well as ours, display partial eclipses and inverse O’Connell effects, with Max I fainter than Max II. Among possible spot models, a cool spot on either of the component stars and its variability with time permit good light-curve representations for the system. A total of 194 eclipse timings over 81 yr, including our five timings, were used for ephemeris computations. We found that the orbital period of the system has varied due to a periodic oscillation overlaid on an upward parabolic variation. The continuous period increase at a fractional rate of +1.3 × 10-10 is consistent with that calculated from the Wilson & Devinney code and can be interpreted as a thermal mass transfer from the less massive to the more massive secondary star at a rate of 6.6 × 10-8 M⊙ yr-1. The periodic component is in satisfactory accord with a light-time effect due to an unseen companion with a period of 28.2 yr, a semiamplitude of 0.007 days, and a minimum mass of M3 sin i3 = 0.25 M⊙, but this period variation could also arise from magnetic activity.