For 39 years, astronomers have been recording how the star BE Lyn brightens and dims. Its light pulsates with a rhythm of several hours, like a lighthouse that never stops blinking. But there is a glitch in this rhythm: sometimes the flashes came a little early, sometimes a little late. The star was moving closer to Earth and then farther away, as if something were dragging it in a circle.

It turns out that BE Lyn is not alone. An invisible companion orbits around it, and the orbit of this pair is stretched to the limit. At the farthest point, the distance between them is hundreds of times greater than at the closest point. For comparison, Earth's orbit is almost circular, but here it is like a highly squashed oval. When the star whips past its companion, its pulse noticeably speeds up.

The companion is heavy — at least 2.5 solar masses, but no light is seen from it. Such a massive yet dark object is most likely a black hole. If confirmed, it would be the closest known to Earth. Astronomers have a rare opportunity to study how strong gravity affects a star's rhythm, and possibly to discover a new way to find black holes that reveal themselves only through gravity.