Every 15 hours, in a constellation 57 light-years away, a miniature solar eclipse takes place. A tiny white dwarf — a star the size of Earth but heavier than the Sun — ducks behind the disk of a red dwarf. For astronomers, this is not just a pretty coincidence but a way to peek into the pair's past.

Once, the primary star in this system swelled into a red giant and engulfed its neighbor. The neighbor, braking against the gas, spiraled inward, and the giant's envelope was stripped away. What remains is what we see: a dense core — the white dwarf — and a surviving red dwarf on a tight orbit. Such pairs are called post-common-envelope systems.

The new pair, TIC-460388167, was found using data from the TESS space telescope and ground-based spectrographs. The eclipses allowed astronomers to pin down the orbital inclination — 89 degrees, meaning we see the system almost edge-on. The orbital period is 0.636 days. The red dwarf turns out to be synchronized: it always shows the same face to the white dwarf, just like the Moon to Earth. This was confirmed — for the first time for such a pair — from the profile of spectral lines.

Such systems are natural laboratories. They show how stars survive a common envelope and what is left afterward. The more such pairs we measure, the clearer one of the most violent stages in the lives of binary stars becomes.