# Properties and nature of Be stars 31. The binary nature, light variability, physical elements, and emission-line changes of HD~81357

## TL;DR

This paper presents a detailed analysis of the binary star HD 81357, revealing its physical properties, orbital characteristics, and evolutionary state, and highlighting its phase of mass transfer after mass-ratio reversal.

## Contribution

First detailed investigation of HD 81357 as a hot emission-line binary with Roche-lobe filling secondary, including spectral, photometric, and evolutionary analysis.

## Key findings

- HD 81357 is a double-lined spectroscopic binary with an orbital period of 33.77445 days.
- The system is in the final slow phase of mass transfer after mass-ratio reversal.
- No cyclic light variations longer than the orbital period were observed.

## Abstract

Reliable determination of the basic physical properties of hot emission-line binaries with Roche-lobe filling secondaries is important for developing the theory of mass exchange in binaries. It is not easy, however, due to the presence of circumstellar matter. Here, we report the first detailed investigation of a new representative of this class of binaries, HD~81357, based on the analysis of spectra and photometry from several observatories. HD~81357 was found to be a double-lined spectroscopic binary and an ellipsoidal variable seen under an intermediate orbital inclination of $\sim(63\pm5)^\circ$, having an orbital period of 33\fd77445(41) and a~circular orbit. From an automated comparison of the observed and synthetic spectra, we estimate the component's effective temperatures to be 12930(540)~K and 4260(24)~K. The combined light-curve and orbital solutions, also constrained by a very accurate Gaia Data Release 2 parallax, give the following values of the basic physical properties: masses $3.36\pm0.15$ and $0.34\pm0.04$~\Mnom, radii $3.9\pm0.2$ and 13.97\pm0.05$~\Rnom, and a~mass ratio $10.0\pm0.5$. Evolutionary modelling of the system including the phase of mass transfer between the components indicated that HD~81357 is a~system observed in the final slow phase of the mass exchange after the mass-ratio reversal. Contrary to what has been seen for similar binaries like AU~Mon, no cyclic light variations were found on a~time scale an~order of magnitude longer than the orbital period. 243,1 15%