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 \pm 5 ) ^ { \circ } , having an orbital period of 33 \aas@@fstack { d } 77445 ( 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 \pm 0.15 and 0.34 \pm 0.04 \mathcal { M } ^ { \mathrm { N } } _ { \odot } , radii 3.9 \pm 0.2 and 13.97 \pm 0.05 \mathcal { R } ^ { \mathrm { N } } _ { \odot } , and a mass ratio 10.0 \pm 0.5 .
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 .