An international team of astronomers has utilized the Zwicky Transient Facility (ZTF) and the Transiting Exoplanet Survey Satellite (TESS) to investigate a binary system designated ZTF J0007+4804. As a result, they have found that ZTF J0007+4804 is the first hot subdwarf-white dwarf system discovered that produces dwarf nova outbursts. The finding is reported in a paper published May 4 on the arXiv preprint server.
ZTF J000742.62+480414.51, (ZTF J0007+4804 for short) was first reported in 2019 as a subdwarf candidate. Three years later, it was reclassified as an ellipsoidal hot subdwarf-white dwarf (WD) binary candidate with an orbital period of approximately 1.8 hours.
Now, the new study based on ZTF and TESS data, conducted by a group of astronomers led by Eric Stringer of the University of Hamburg in Germany, has provided more insights into the properties of ZTF J0007+4804, confirming its hot subdwarf binary nature.
Observations and data collection
“The system was observed as part of the ZTF public survey in the g, r, and i bands with 30-second exposures, which were taken between May 2018 and February 2024 for a total of 2,249 data points. (…) We also used 120 second cadence data from TESS. TESS observed ZTF J0007+4804 (TIC 201736330) in sectors 17, 57, and 84 between October 2019 and October 2024 for a total of 47,104 data points,” the researchers write in the paper.
The study found that ZTF J0007+4804 consists of an accreting white dwarf and a B-type hot subdwarf acting as a donor. The subdwarf has an effective temperature of 23,500 K and a mass of about 0.42 solar masses, while the WD is only slightly more massive—its mass is estimated to be 0.48 solar masses. The orbital period of the system was calculated to be approximately 1.81 hours.
Dwarf nova outbursts and system dynamics
Furthermore, it turned out that ZTF J0007+4804 experiences dwarf nova (DN) outbursts. In the case of ZTF J0007+4804, these DN outbursts are of the SU UMa-type with a recurrence time of about nine days. The astronomers explained that this subtype is characterized by the presence of superoutbursts in between regular outbursts, which occur with a lower frequency than the normal outbursts, and last for one to two weeks.
The researchers underlined that ZTF J0007+4804 is therefore the first known hot subdwarf-WD system exhibiting periodic dwarf nova outbursts. It is also the fourth known subdwarf-WD binary undergoing Roche lobe overflow.
Future evolution and potential merger
Based on the collected data, the authors of the paper estimate that the mass transfer rate of ZTF J0007+4804 is at a level of 1.6 × 10−11 solar masses per year. They predict that in about 226 million years, the systems will merge due to gravitational wave emission, which will likely produce a single massive hydrogen-deficient white dwarf. However, they do not exclude a thermonuclear explosion scenario.
“The system has likely formed from a main sequence binary with component masses of over two solar masses and will likely merge into a single white dwarf, but a thermonuclear explosion cannot be ruled out,” the scientists conclude.
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Publication details
Stringer et al, Discovery of the first outbursting hot subdwarf binary: ZTF J0007+4804, arXiv (2026). DOI: 10.48550/arxiv.2605.03010
Journal information:
arXiv
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First outbursting hot subdwarf binary discovered (2026, May 16)
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