A long-term photometric study of the near-contact binary RU ursae minoris

New CCD photometric observations of the near-contact binary RU UMi were obtained from 1999 December to 2000 April and later during 2003 March. All available times of minimum light, including our own measurements, have been examined and indicate that the orbital period of the system has experienced a continuous period decrease with a rate of (dP/dt)(obs) = -1.6 x 10(-8) d yr(-1). This period decrease cannot be explained trivially by mass transfer from the more massive primary to the less massive secondary, because our light-curve synthesis indicates that RU UMi is in a semidetached configuration with the primary star filling 95% of its inner Roche lobe and the secondary filling its limiting lobe completely. We present the case that the observed period change may be produced by a combination of AML due to magnetic braking with a rate of (dP/dt)(mb) = -1.0 x 10(-7) d yr(-1) and of mass transfer from the secondary to the primary at a rate of M-2 = -6.0 x 10(-8) M-circle dot yr(-1). Historical light curves of RU UMi over a 32-yr interval display variably asymmetrical secondary minima. From analyses of velocity and light curves and of the light curves alone, the light variations are best represented by a variable cool spot that has consistently existed on the inner hemisphere of the secondary component. There is a weak indication that the hot star is no longer a main sequence one.