  We obtained high-resolution spectra and multicolor photometry of V1082 Sgr to
study the donor star in this 20.8 hr orbital period binary, which is assumed to
be a detached system. We measured the rotational velocity (v sin i = 26.5+/-2.0
km/s), which, coupled with the constraints on the white dwarf mass from the
X-ray spectroscopy, leads to the conclusion that the donor star barely fills
70% of its corresponding Roche lobe radius. It appears to be a slightly evolved
K2-type star. This conclusion was further supported by a recently published
distance to the binary system measured by the Gaia mission. At the same time,
it becomes difficult to explain a very high (> 10e-9 Msun/yr) mass transfer and
mass accretion rate in a detached binary via stellar wind and magnetic
coupling.
