Two White Dwarves, Five Shared Properties, One New Class of Star
Astronomers have defined a new class of star remnants — and it took finding two of them to do it.
A team at the Institute of Science and Technology Austria (ISTA) has published in Astronomy & Astrophysics a paper establishing that two unusual white dwarf merger remnants — named Gandalf and Moon-Sized — share five properties that, together, are unprecedented: ultra-massive, highly magnetic, rapidly rotating, companionless, and emitting X-rays. The last property is the most striking. X-ray emission from white dwarfs is almost always a signature of accretion from a binary companion. These two objects have no companion. They emit X-rays anyway.
The five-property combination is a first. The researchers argue that two examples are sufficient to define a new class, and that more examples may now be found deliberately rather than by chance.
The objects themselves are odd on their own terms. Gandalf rotates on its axis every six minutes — far faster than any known white dwarf binary orbit, which maxes out at 80 minutes. If Gandalf had a companion, its magnetic field should synchronize its rotation with the orbital period. The six-minute spin is incompatible with that scenario. Its X-ray emissions are also 100 times brighter than Moon-Sized's. The merger event that formed Gandalf happened roughly 60 to 70 million years ago, about the time dinosaurs were still dominant on Earth.
Moon-Sized (formally catalogued as ZTF J1901+1458) is older — its merger happened around 500 million years ago, roughly seven to eight times older than Gandalf. Its effective temperature is 28,015 Kelvin, measured to a precision of ±20 K, which tells you something about how carefully this object has been characterized.
Both remnants show hydrogen emission spectra with a double-peaked signature — what the research team describes as "cat ears." In Gandalf's case, the peaks alternate on the six-minute spin period, which the team interprets as a half-ring of material asymmetrically captured by the object's magnetic field. The phrase from the ISTA press release is direct: "We have never seen anything like that before in any white dwarf."
The lead researcher is Ilaria Caiazzo, assistant professor at ISTA. The first author on the Gandalf paper is Andrei Cristea, a PhD student. The paper appeared in Astronomy & Astrophysics on February 10, 2026, with an arXiv preprint also available.
The practical implication the team is making: if you know what to look for, you can now search archives of existing sky surveys for other candidates rather than waiting to stumble onto them. Two examples define a class; a class defines a search template.
ISTA press release: https://ista.ac.at/en/news/twos-company-new-class-of-star-remnants/
Paper (A&A, DOI): https://doi.org/10.1051/0004-6361/202556432
arXiv preprint: https://arxiv.org/abs/2509.03216