Several years ago, Burdge noticed a light curve — a graph of brightness over time — that looked nothing like the bell curve of a typical supernova. It was triangular, and it kept repeating.

"Stars don't make triangular waveforms like that," Burdge recalls.

The team first considered a black-widow binary — a system in which a neutron star slowly strips material from a smaller companion. But the brightness of ZTF J0440+2325 was not flickering back and forth the way black-widow signals do.

Graduate student Aaron Householder, of MIT's Department of Earth, Atmospheric and Planetary Sciences, helped measure the source's wobble: the small side-to-side motion that reveals the gravitational pull of an orbiting partner.

"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 orbited by a brown dwarf. The wobble was too small in amplitude to be anything else."

What is the star actually eating?

The two objects are close enough that the star's gravity strips gas directly off the brown dwarf's surface. In typical accretion, the matter spirals into a disk before falling in. Here, Burdge says, the star is large enough in size that matter "pummels directly onto the surface at very high speeds, like an asteroid hitting the Moon."

The team ran computer simulations of particles on the brown dwarf to test where they would end up under the system's gravity. "When we track those test particles, we see they fall straight onto the star's surface," Householder says. "This is the first time we've caught a low-mass star actively accreting from another low-mass object."

The transfer rate is small by cosmic standards: about 1/100,000 of an Earth's mass per year, which the team describes as roughly 40 million dump-truck loads of material, or about 1.3 trillion one-pound burritos every second. Yet that trickle, multiplied over billions of orbits, can persist for the lifetime of the system.

How does the triangle fit in?

Burdge compares the slow transfer to a continuous fireball on one face of the star. "It's like a continuous fireball onto one of the objects, and as one orbits the other, that hotspot comes in and out of view," he explains. "The peak of the triangle signal is when you're looking right at the fireball."

The team now plans to hunt for similar triangular signals in other surveys, in case slow-feeding pairs are not as rare as they look.

"It's inspiring a lot of new searches on our part," Householder says. "I think we will learn a lot about a different kind of way that planets and brown dwarfs interact with their host stars."

MIT co-authors include Kaitlyn Shin, Saul Rappaport, Joheen Chakraborty, and Emma Chickles. Collaborators came from Caltech, the University of Hawaii, the Instituto de Astrofísica de Canarias and the Universidad de La Laguna in Spain, and the Harvard and Smithsonian Center for Astrophysics. Funding came in part from the U.S. National Science Foundation.