A Dance of Two Stars 57 Light-Years Away express
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.
Original abstract
as written by the authors arXiv:2604.07527
Short-period white dwarf+main-sequence binaries are Post-Common-Envelope Binaries (PCEB) that have survived a common envelope phase. Such systems, if detached and eclipsing, enable precise measurements of the constituent stars, providing a unique opportunity to probe the effects of the common envelope phase on the system. We report the discovery of one such nearby (57 pc) system, TIC-460388167, using a combination of multi-band photometric light curves and spectroscopic radial velocities. In addition to eclipses, the system exhibits a continuously variable light curve that we model as a combination of ellipsoidal variations and star spots. We determine a period $$P$$=0.63596258$$\pm$$0.00000012 d and inclination $$i$$=89.0$$\pm$$0.4 deg. The best-fitting model specifies a white dwarf with T$$_1$$=7607$$\pm$$127 K and radius R$$_1$$=0.0131$$\pm$$0.0003 $$R_\odot$$, which is eclipsed by a T$$_2$$=3151 $$\pm$$ 59 K, R$$_2$$=0.327$$\pm$$0.006 $$R_\odot$$ M dwarf. The white dwarf mass is 0.61$$\pm$$0.04 M$$_\odot$$. We present the first velocity resolved profile for a PCEB secondary and show that the rotation of the M-dwarf is synchronous with the orbital period, as expected. We compare the constituent stars to other PCEB systems and find TIC-460388167A is one of the coolest known white dwarfs in such systems. TIC-460388167 is among the longest period eclipsing PCEB systems known.
The orbital period of the pair is 15 hours and 16 minutes. In that time, the white dwarf manages to completely hide behind the red dwarf and reappear.