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We present the orbital period variability and evolutionary status of the WUMa-type binary system V864 Mon from accurately measured fundamentalparameters. New $BV$ photometric observations of this system were performed in2019 January and 2022 January, and the first high-resolution spectroscopicobservations were carried out on three nights between 2019 January and March. Atotal of 29 times of minimum light were collected to examine the behavior ofthe orbital period. Our analysis of these timings indicates a continuous periodincrease at a rate of $+$2.62$\times$10$^{-7}$ d yr$^{-1}$ over the past 20years, which can be interpreted as a mass transfer from the less massiveprimary to the secondary component with a rate of 1.22$\times$10$^{-7}$M$_\odot$ yr$^{-1}$. We measured the radial velocities (RVs) for bothcomponents, and determined the effective temperature and projected rotationalvelocity of the more massive secondary star to be $T_{\rm eff,2}$ = 5450 $\pm$94 K and $v_2 \sin i$ = 192 $\pm$ 40 km s$^{-1}$, respectively, from thecomparison of the observed spectrum at the primary minimum and the theoreticalmodels. The individual masses and radii of both components were determined froma simultaneous analysis of the light and RV curves, which are $M_1$ = 0.34$\pm$ 0.02 M$_\odot$, $R_1$ = 0.69 $\pm$ 0.01 R$_\odot$, and $M_2$ = 1.06 $\pm$0.04 M$_\odot$, $R_2$ = 1.16 $\pm$ 0.02 R$_\odot$, respectively. Our resultsindicate that V864 Mon is a W-subtype of W UMa stars with time-varying spotactivity. The positions in the mass-luminosity and mass-radius diagramsindicate that the secondary star belongs to the main sequence region, while thehotter primary is located beyond the terminal-age main sequence.

Photometric and Spectroscopic Properties of the Eclipsing System V864 Monocerotis