2023PASJ...75.1136P

We present the orbital period variability and evolutionary status of the W UMa-type binary system V864 Mon from accurately measured fundamental parameters. New BV photometric observations of this system were performed in 2019 January and 2022 January, and the first high-resolution spectroscopic observations were carried out on three nights between 2019 January and March. A total of 29 times of minimum light were collected to examine the behavior of the orbital period. Our analysis of these timings indicates a continuous period increase at a rate of +2.62 × 10^–7 d yr^–1 over the past 20 years, which can be interpreted as a mass transfer from the less massive primary to the secondary component with a rate of 1.22 × 10^–7 M_⊙ yr^–1. We measured the radial velocities (RVs) for both components, and determined the effective temperature and projected rotational velocity of the more massive secondary star to be T_eff,2 = 5450 ± 94 K and v₂sin i = 192 ± 40 km s^–1, respectively, from the comparison of the observed spectrum at the primary minimum and the theoretical models. The individual masses and radii of both components were determined from a simultaneous analysis of the light and RV curves, which are M₁ = 0.34 ± 0.02 M_⊙, R₁ = 0.69 ± 0.01 R_⊙, and M₂ = 1.06 ± 0.04 M_⊙, R₂ = 1.16 ± 0.02 R_⊙, respectively. Our results indicate that V864 Mon is a W-subtype of W UMa stars with time-varying spot activity. The positions in the mass-luminosity and mass-radius diagrams indicate that the secondary star belongs to the main-sequence region, while the hotter primary is located beyond the terminal-age main sequence.