Abstract Complete BV light curves of the W UMa type binary RT LMi are presented. From the observations, four times of minimum light were determined. Based on the new times of minimum light and those collected from the literature, changes in the orbital period of the system were found and analyzed with Kalimeris et al.'s method. The result shows that the orbital period possibly oscillates with a cycle of about 64 years and an amplitude of 1.2 $\times$ 10-6 days. The present CCD photometric observations reveal that the light curves are obviously asymmetrical, and show a positive O'Connell effect, while the light curves obtained in 1982 exhibit a negative O'Connell effect. The present light curves were analyzed by means of the latest version of the Wilson-Devinney code, which was also used to correct the photometric effects, including the distortion on the radial-velocity curves obtained by Rucinski et al. The following absolute dimensions have been derived: M1 = 1.28±{0.08}~M⊙, M2 = 0.48±{0.06}~M⊙, R1 = 1.28±{0.06}~R⊙, R2 = 0.83±{0.05}~R⊙, L1 = 1.88±{0.12}~L⊙, L2 = 0.77 ±{0.08}~L⊙, and A = 2.64±{0.02}~R⊙. The asymmetry of the light curves can be explained by a model with a cool spot on the secondary component. The orbital period modulation can be reproduced by a magnetic activity cycle model of the secondary component with Δ{J} = 5.6× 1046 g cm2 s-1, Δ{Ω}/Ω = 8.8 × 10-4, and B = 5.1 x 103G.

Keywords star: contact binary -- star: individuals: RT LMi - star: magnetic activity

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