An international team led by Indian astronomers has identified a pair of white dwarfs locked in an extraordinarily tight orbit that is shrinking faster than that of two other well-known systems of the same type. The binary, known as eRASSU J060839.5–704014 or eRASSU J0608, completes an orbit in just 374.15 seconds, or about 6.2 minutes. Researchers tracked the system’s orbital motion for three and a half years using observations from NASA’s NICER telescope and the Chinese-led Einstein Probe mission, together with archival data from the European Space Agency’s XMM-Newton observatory. Their findings, published in The Astrophysical Journal Letters titled Rapid Orbital Decay in the Ultracompact Double-degenerate Binary eRASSU J060839.5–704014 on August 10, 2026, show that the system is among the most rapidly evolving ultracompact double-white-dwarf binaries currently known. The researchers say its predictable gravitational-wave signal could also make it a promising verification source for future space-based observatories such as the planned Laser Interferometer Space Antenna, or LISA.
The two white dwarfs orbit each other in just 374 seconds
White dwarfs are the dense stellar remnants left behind when stars such as the Sun exhaust their nuclear fuel and shed their outer layers. In an ultracompact binary, two such remnants can orbit one another at an extremely small separation, producing orbital periods measured in minutes rather than days or years. eRASSU J0608 completes one orbit every 374.15013 seconds, equivalent to about 6.24 minutes. The system was first identified as a supersoft X-ray source in observations from eROSITA, the X-ray instrument aboard the Spektrum-Roentgen-Gamma mission. Follow-up observations established that the source was consistent with a double-degenerate ultracompact binary in the foreground of the Large Magellanic Cloud. The short orbital period is accompanied by a strongly modulated X-ray signal. Earlier observations found pulsations at roughly 374 seconds, while the new study used the timing of those X-ray pulses to follow changes in the binary’s orbit over time.
Material from one white dwarf may be striking the other directly
The system belongs to a rare class of ultracompact binaries in which material transferred from one white dwarf can reach its companion without first forming a conventional accretion disc. This configuration is known as direct-impact accretion. The researchers’ X-ray observations support this interpretation. The system produces a supersoft X-ray spectrum, with the new study measuring thermal components corresponding to temperatures of about 126 and 144 electronvolts. Those temperatures are equivalent to roughly 1.5 million and 1.7 million kelvin. The direct-impact interpretation also provides an explanation for the system’s strong X-ray modulation. As material from the donor white dwarf reaches the surface of its companion, the impact region can become extremely hot and emit X-rays. The bright phase changes in temperature during the orbit, which the researchers interpreted as evidence for a structured emitting region with significant temperature differences across it.
Its orbit is shrinking faster than those of HM Cnc and V407 Vul
To measure how the binary is evolving, the researchers combined X-ray timing observations from NICER and Einstein Probe with archival XMM-Newton observations. By connecting these observations over a baseline of about three and a half years, they obtained a coherent timing solution for the system. The measured orbital period is decreasing at a rate of approximately 4.7 × 10⁻¹¹ seconds per second. That may sound tiny, but over astronomical timescales it represents an unusually rapid change for a binary system. The researchers found that the measured orbital decay is greater than that observed in HM Cnc and V407 Vul, two other well-studied ultracompact double-degenerate binaries. The rapid decay is consistent with the system losing orbital energy and angular momentum through gravitational radiation. In their analysis, the researchers assume that gravitational-wave angular-momentum loss is the primary driver of the observed orbital evolution. Under that assumption, they infer a chirp mass of about 0.43 times the mass of the Sun.
The system may be among the most massive of its class
The chirp mass is a parameter that describes a binary’s component masses and plays an important role in determining the evolution and gravitational-wave signal of an inspiralling system. For eRASSU J0608, the inferred value of about 0.43 solar masses places it among the most massive known systems in this particular class, according to the researchers. The authors describe eRASSU J0608 as one of the most rapidly evolving ultracompact double-degenerate binaries currently known. Its unusually fast orbital evolution makes it useful for studying the short-lived stages through which extremely compact stellar binaries evolve. The system is also potentially important for gravitational-wave astronomy. Its orbital period is short enough to place its expected gravitational-wave emission in the low-frequency regime targeted by future space-based detectors rather than the frequency range of today’s ground-based observatories.
eRASSU J0608 could become a verification source for LISA
The researchers say eRASSU J0608 is a promising verification source for future low-frequency gravitational-wave observations. A verification source is a binary whose gravitational-wave signal can be predicted from independent observations, allowing researchers to compare that prediction with what a future gravitational-wave detector actually measures. One mission designed to study low-frequency gravitational waves is ESA’s Laser Interferometer Space Antenna (LISA). The mission will use three spacecraft separated by millions of kilometres to measure tiny changes produced by passing gravitational waves. ESA currently plans LISA for launch in 2035.For eRASSU J0608, however, an important uncertainty remains: its distance has not yet been firmly established. The earlier discovery study placed the system in the direction of the Large Magellanic Cloud and concluded that it lies in the foreground of the galaxy, but determining its distance more precisely would improve estimates of the system’s physical properties and expected gravitational-wave signal. The IUCAA-led team plans to continue observing the binary with X-ray telescopes and search for a visible-light counterpart. Those observations could help researchers determine its distance and component masses more precisely, allowing them to refine predictions for the gravitational-wave signal that future space observatories could measure.
