Spectroscopic and Decade-long Photometric Observations of the Contact Binary V2790 Ori: Evidence for a Brown Dwarf Companion and a Solar-like Magnetic Activity Cycle

Abstract We present 22 sets of light curves and one radial velocity curve for the W UMa-type total eclipse contact binary system V2790 Ori, derived by combining all available public photometric data, the photometric data in previous studies, and our own spectroscopic and decade-long photometric observations. Our simultaneous analysis of the light curves and radial velocity curve shows that V2790 Ori is a W-subtype contact binary with a mass ratio of q = 0.322(±0.001) and a shallow contact degree of 14.8(±0.6)%. The orbital period analysis based on 445 eclipsing minima reveals a secular decrease at a rate of P ̇ = − 3.18 ( ± 0.75 ) × 1 0 − 8 days yr − 1 , superimposed with a cyclic variation with an amplitude of A = 8.98(±2.19) × 10 −4 days and a period of P 3 = 7.44(±0.52) yr. The secular decrease is caused by angular momentum loss via magnetic braking, while the cyclic period variation is explained by the light-travel time effect due to a third body, which is likely to be a brown dwarf. Furthermore, our analysis indicates a mass transfer from the more massive component to the less massive one at a rate of 1.22(±0.29) × 10 −8 M ⊙ yr −1 . A model with a cool spot on each component was adopted to fit the O’Connell effect observed in the light curves. Since the O’Connell effect varies over time, we identified a solar-like magnetic activity cycle with a period of approximately 5.4 yr by analyzing the magnitude difference (Δ m ) at the two light maxima and the O’Connell effect ratio. In addition, evolutionary analysis suggests that V2790 Ori is a newly formed contact binary that evolved from a detached phase into the present contact configuration.