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The Light and Period Variations of the Eclipsing Binary AA Ursae Majoris
We present new multiband CCD photometry for AA UMa made on 8 nights between
January and March 2009; the 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 yrs, 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 1.310 is consistent with
that calculated from the W-D code and can be interpreted as a thermal mass
transfer from the less to the more massive secondary star at a rate of
6.610 M yr. The periodic component is in
satisfactory accord with a light-time effect due to an unseen companion with a
period of 28.2 yrs, a semi-amplitude of 0.007 d, and a minimum mass of =0.25 but this period variation could also arise from
magnetic activity.Comment: 23 pages, including 5 figures and 8 tables, accepted for publication
in PAS
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