We present new multiband CCD photometry for AA UMa made on 8 nights between January and March 2009 ; 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 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 + 1.3 \times 10 ^ { -10 } 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 6.6 \times 10 ^ { -8 } M _ { \odot } 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 yrs , a semi-amplitude of 0.007 d , and a minimum mass of M _ { 3 } \sin i _ { 3 } =0.25 M _ { \odot } but this period variation could also arise from magnetic activity .