2025RAA....25b5006W

Using ground-based telescopes, the multi-color photometric observations of the contact binary EF Boo were obtained in 2020, 2023, and 2024. Combining these with 7-sectors of light curves from TESS data, the variations of the O'Connell effect in continuous time and shapes of light curves over several years were identified. Three sets of typical light curves were analyzed to determine the photometric solutions via the Wilson-Devinney program. Considering the spectroscopic mass ratio of q = 0.53, these photometric solutions suggest that EF Boo is a W-type W UMa contact binary with the averaged filling factor of f = 22.26%, a small temperature difference, and a cool spot on the primary component. If the variations of the O'Connell effect are due to the magnetic activity of this cool spot, the longitudinal location varied from 50$\mathop{.}\limits^{\unicode{x000b0}}$4 to 302$\mathop{.}\limits^{\unicode{x000b0}}$7 over the time interval of 1434 days. Based on all CCD minimum times from ground-based telescope and TESS data, the O - C curve was also analyzed. A cyclic oscillation (A₃ = 0.00575 days, T₃ = 27.8 yr) superimposed on a secular increase (dP/dt = 6.74 × 10^–8 day yr^–1) was discovered for the first time. The successive increase is possibly a result of mass transfer from the less massive star to the more massive one. The cyclic oscillations were possibly explained by the light-travel time effect via a third body or the magnetic activities. From the short cadence observations from TESS, we also calculated the value of the O'Connell effect and O - C value for each cycle and found no correlation between the O'Connell effect and O - C over nearly 30 days across different sectors.