HW Vir systems are rare evolved eclipsing binaries composed of a hot compact star and a low-mass main sequence star in a close orbit. These systems provide a direct way to measure the fundamental properties, e.g. masses and radii, of their components, hence they are crucial in studying the formation of subdwarf B stars and low-mass white dwarfs, the common-envelope phase and the pre-phase of cataclysmic variables. Astronomers only know about two dozen of this type of binary system.

HS2231+2441, an HW Vir type, is a binary system composed of a low-mass white dwarf, with only 0.2-0.3 of a solar mass, with an effective temperature of 28,500 K, and a brown dwarf with 36-46 Jupiter masses. The binary system has an orbital period of approximately 3 hours. HS2231+2441 is the least massive HW Vir system known.

Astronomers analysed HS2231+2441 using data from Picos do Dias Observatory in Brazil and the ISIS spectrograph on the William Herschel Telescope. The high-quality spectroscopic data were crucial to derive, with high precision, the main parameters of the system, e.g., effective temperature, surface gravity and metallicity, and combined with photometry, the mass and radius of each component. With this information, it is possible to study the current evolutionary status of the system.

The primary star, a white dwarf, evolved more rapidly because it is more massive than the secondary star, a brown dwarf. When the primary ascended the red-giant branch, it began to transfer material to its companion. This mass transfer occurred rampantly in a short time scale and eventually, the primary engulfed the secondary in a 'big hug'.

The secondary object was attracted and engulfed by the primary object's extended atmosphere, called its envelope, within which it began orbiting. During this process of attraction, the secondary object lost orbital angular momentum (rapid translation of its centre of mass) owing to collision and friction with the primary object's envelope, converted into kinetic energy of the envelope.

When the energy transferred by the secondary object reached the point where it exceeded the gravitational force keeping the envelope anchored to the primary object's core, a huge ejection of matter from the system occurred, stripping the primary object down to its exposed helium core.

Because the ejected matter corresponded to a large proportion of the primary object's mass, it could be said to have died prematurely; it was unable to burn more core helium and generate its own light. Therefore, the primary can be considered as an extremely low-mass white dwarf.