  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.
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