How did a 'weird triangle' give it away?

The story began several years ago, when Burdge was combing through data from the Zwicky Transient Facility (ZTF), a camera on a telescope at the Palomar Observatory in California that scans the sky for sudden changes in brightness — the hallmarks of supernovas, gamma-ray bursts, or colliding neutron stars.

Supernova light curves trace a bell shape: a star brightens, then fades. The signal Burdge noticed instead resembled a triangle, and it repeated over and over.

"I remember first looking at this and thinking: Stars don't make triangular waveforms like this," he recalls.

At the time, his team was chasing a different signal from a "black widow binary" — a system where a dense, spinning neutron star slowly consumes a smaller companion star. Burdge wondered whether the triangle came from a black widow too. But the light didn't wobble the way those systems do, where a lightweight object whips around a much heavier one.

"We weren't seeing that whipping back and forth here," Burdge says. "It didn't make any sense. We couldn't explain what this was."

Their hunch: the signal came from a more balanced pair, two objects of similarly low mass gently orbiting each other. "But we never had any proof. And this weird triangle just sat for years," Burdge says.

What clinched the identification?

Recently, Burdge and graduate student Aaron Householder of MIT's Department of Earth, Atmospheric and Planetary Sciences revisited the mystery. They pinpointed the source's location — roughly 300 light-years away — and trained multiple telescopes on it, measuring properties including its wobble.

The wobble was far smaller than in black widow systems, but not negligible.

"That was the real clincher for this system," Householder says. "When we measured that wobble, we found we were not seeing a black widow. This was a low-mass star that's orbited by a brown dwarf. The wobble was too small in amplitude to be anything else."

The team then ran simulations of accretion — the process in which one object gravitationally pulls material from another. Accretion is familiar around black holes and neutron stars, which are massive but compact, drawing matter into a swirling disk. Here the devourer is a star, which is relatively large.

"So matter just pummels directly onto the surface, at very high speeds, like an asteroid hitting the moon," Burdge explains.

The simulations tracked test particles on the brown dwarf and how they should move over time according to the laws of physics. "When we track those test particles, we see they indeed fall right onto the surface of the star," Householder says. "This is the first time we've caught a low-mass star actively accreting from another low-mass object."

The simulations also solved the triangle puzzle. Material streams steadily from the brown dwarf onto the star, creating a persistent hot spot — a "continuous fireball," in Burdge's words — that swings in and out of view as the pair orbits. The triangle's peak occurs when the fireball faces Earth directly.

Why does it matter?

One system is a single data point, and the researchers caution there is still much to learn about how such systems form and evolve. But the find already shows that star–planet and star–brown-dwarf interactions are not limited to stable coexistence or wholesale engulfment. A third mode exists: long, slow feeding.

The team is now searching for similar slow-feeding systems nearby. "It's inspiring a lot of new searches on our part," Householder said. "I think we're going to learn a lot about a different kind of way that planets and brown dwarfs interact with their host stars."

The paper, by Aaron Householder et al., appears in Nature Astronomy (2026), DOI: 10.1038/s41550-026-02992-6.