Two glowing embers left over from a fire whirl in the darkness, gradually drawing closer. With each turn, they lose tiny bits of energy that seep into the invisible ripple of space. That is exactly what the system eRASSU J060839.5−704014 looks like — a pair of white dwarfs (superdense stellar remnants) separated by a distance comparable to Earth's diameter. Their dance lasts only 374 seconds — quicker than a bullet fired from a rifle can cross the sky.

Using three X-ray telescopes (NICER, Einstein Probe, and archival data from XMM-Newton) astronomers tracked the system for several years. They managed to build a precise light curve and noticed that a bright spot on the surface of one dwarf cools as it moves. This suggests that the radiation comes not from the whole star but from a small region where matter falls from the companion. Such systems are called direct-impact — the collision happens head-on, without forming a disk.

The rate of orbital shrinkage turned out to be record-breaking for such pairs. It exceeds the rates of the famous binaries HM Cancri and V407 Vul. If all the lost energy goes into gravitational waves (ripples in spacetime), then the total mass of the system is about 0.43 solar masses. This makes eRASSU J060839 one of the most massive among known ultracompact binaries. For future low-frequency gravitational wave detectors like LISA, this object will be an ideal calibration target: its signal can already be predicted with high precision.

The embers continue to close in, and their dance accelerates. Each year, the orbit becomes a little tighter, and the ripple in space becomes a little more noticeable to sensitive instruments.