In this paper, we present the results of a study of a long-period eclipsing binary system V891 Mon having a period of P = 31. 42050ᵈ. The presented study is based on spectral data obtained during 2020–2022 on the echelle spectrograph HRS of the SALT telescope and TESS photometric data. The constructed radial velocity curve is based on 11 spectra covering all phases of this binary system. The orbital parameters of this binary system have been determined for the first time. The constructed model for the light curve according to TESS data converges with an accuracy of 0. 01%. The effective temperatures of both components, their rotational velocities, as well as the metallicity of the system and the interstellar reddening value are directly determined from the spectra. Our analysis of photometric and spectral data allowed us to directly calculate the luminosities of both components, their radii and masses with accuracy of less than one percent. A comparison with the evolutionary tracks shows that the age of the system is 325 Myr, component A has already left the main sequence (MS), and component B is still on MS. Both components are not synchronized, and the system is not circularized. The kinematic characteristics, together with the metallicity determined in this work, indicate that the V891 Mon system is a typical representative of the “kinematically cold” component of a thin disk, which is consistent with its evolutionary status as a young system. Despite the large eccentricity, the orbit of the system is oriented to the line of sight in such a way that it can only be determined using spectral data. The work is partially based on a talk presented at the Modern Stellar Astronomy 2025 conference.
Kniazev et al. (Mon,) studied this question.