HW Vir's Companion: a M-type Dwarf, or maybe a giant rotating spherical Mirror?

TL;DR

This study uses high-resolution spectra and radial velocity measurements to determine that HW Vir's unseen companion is likely an M-type main sequence star, based on its mass and radius.

Contribution

It introduces a method to characterize an unseen companion in a binary system without direct spectral detection of the secondary.

Findings

Companion's mass and radius consistent with M-type main sequence star

Reflected light detected near secondary eclipse

Radial velocity measurements enable companion characterization

Abstract

From optical high-resolution spectra the nature of the unseen companion of HW Vir is determined without detection of any spectral features originating from the secondary itself. Using radial velocity measurements from the primary hot subdwarf B star and from weak additional absorption lines detected close to the secondary eclipse, probably caused by reflected light off the surface of the secondary, the mass and radius of the companion is determined. The values are consistent with those of a M type main sequence star.