Photometric and spectroscopic properties of the eclipsing system V864 Monocerotis.

This article focuses on the orbital period variability and evolutionary status of the W UMa-type contact binary system V864 Monocerotis (V864 Mon) through new photometric and spectroscopic observations. The study finds a continuous increase in the orbital period at a rate of +2.62 × 10⁻⁷ days per year, attributed to mass transfer from the less massive primary star (0.34 ± 0.02 solar masses) to the more massive secondary star (1.06 ± 0.04 solar masses) at a rate of 1.22 × 10⁻⁷ solar masses per year. Spectral analysis determined the effective temperature and projected rotational velocity of the secondary star to be 5450 ± 94 K and 192 ± 40 km/s, respectively. Binary modeling reveals that V864 Mon is a W-subtype W UMa system with a fill-out factor of 13% and exhibits time-varying spot activity on the secondary star, which explains observed changes in eclipse depth. Evolutionary analysis places the secondary star on the main sequence, while the primary is beyond the terminal-age main sequence, suggesting the system is in an expanding phase consistent with thermal relaxation oscillation theory.