2025ApJ...987..205K

We present the discovery and analysis of a nearby eclipsing ultracompact accreting binary SMSS J1138-5139, the first well-constrained LISA-detectable Type Ia supernova progenitor. Our time-series optical spectroscopy identifies its orbital period through radial velocity monitoring at ${P}_{\mathrm{orb,RV}}=27.682\,\mathrm{minutes}$, twice the photometric period seen in 2 minute cadence data from TESS Sector 37. We model our optical spectroscopy together with new simultaneous multiband time-series photometry from Gemini to place constraints on the binary parameters. Our light-curve modeling finds that SMSS J1138-5139 contains an M₂ = 0.25 ± 0.01 M_⊙ pre-white-dwarf donor with a massive M₁ = 1.02 ± 0.05 M_⊙ white dwarf accretor at orbital inclination ​​​​​​$i=88\mathop{.}\limits^{\circ }6\pm 0\mathop{.}\limits^{\circ }1$. Based on our photometrically derived system parameters, we expect that gravitational-wave radiation will drive SMSS J1138-5139 to a merger within τ = 5.7 ± 0.3 Myr and result in a Type Ia supernova. Even without a direct merger event, the component masses of SMSS J1138-5139 and active hydrogen accretion suggest that eventual helium accretion will likely also trigger a Type Ia supernova explosion through the dynamically driven double-degenerate double-detonation (D6) channel. We expect LISA to detect the gravitational-wave emission from SMSS J1138-5139 with a signal-to-noise ratio of 7-10 after a 48 month mission.