Observing orbital decay in the ultracompact hot subdwarf binary system ZTFJ213056.71+442046.5

Context. Ultracompact Galactic binary systems emit low-frequency gravitational waves in the millihertz regime. The emission of gravitational waves causes these systems to lose angular momentum, which is detectable by observing the decay of the orbital period of the binary. The system ZTFJ213056.71+442046.5 (ZTF J2130) is an ultracompact binary with an orbital period of 39.34 minutes consisting of a Roche lobe-filling hot subdwarf and a white dwarf companion. Aims. We attempt to measure the orbital decay rate, Ṗ , caused by gravitational wave emission of ZTF J2130 and predict the expected gravitational wave signal for LISA. Methods. High-speed photometry was conducted using the Finger-Lakes-Instrumentation Kepler KL4040FI complementary metal oxide semiconductor (CMOS) camera, mounted on the 1.2-meter Oskar Lühning Telescope at the Hamburg Observatory, as well as the Hamamatsu ORCA-Quest 2 qCMOS camera at the 1.23-meter telescope at the Centro Astronómico Hispano en Andalucía (CAHA) in Spain. ZTF J2130 was observed on six nights between August 2024 and September 2025. The obtained light curves combined with previous high-cadence observations were used to conduct an O − C timing analysis. Additionally, we employed the LISA data analysis tool LDASOFT to model the expected gravitational wave data. Results. We measure a rate of period change of ( − 2.05 ± 0.29)×10 −12 ss −1 . Assuming only gravitational wave emission, the rate of period change corresponds to a chirp mass of (0.42 ± 0.04) M ⊙ . From LDASOFT we predict that LISA will be able to measure the chirp mass with an uncertainty of ∼10%. Conclusions. This work measures the orbital decay with an uncertainty of ≈14% and shows that modern (q)CMOS detectors are well suited for providing precise timing measurements, enabling the measurement of the orbital decay of compact Galactic binaries with high precision even with modest-size telescopes. The derived orbital decay is fully consistent with predictions from spectral and light curve modeling, although the masses are not known with sufficient precision to measure any deviation of the orbital decay from only gravitational waves. We show that future observations with LISA can potentially provide a deviation from only gravitational wave effects, for example due to accretion, if the effect is sufficiently large.