An Intermediate Mass Stripped Star in a 4.2-hour Orbit around a Compact Object

From their discovery as radio pulsars to landmark multi-messenger observations of their mergers, double neutron stars have transformed our understanding of the extreme Universe and the formation of heavy elements (Drout et al. 2017; Abbott et al. 2017). Yet, uncertainties in their formation result in predicted merger rates spanning orders of magnitude (Tauris et al. 2017; Mandel & Broekgaarden 2022). The most critical stage occurs when a star is stripped of its hydrogen-rich envelope, causing the binary orbit to shrink drastically. Despite its fundamental role, the resulting tight binaries comprising a stripped star and neutron star have remained elusive. Here we report a $\sim$$3.4\,M_{\odot}$ stripped star (Ludwig et al. 2026; Drout et al. 2023; Götberg et al. 2023) orbiting an unseen companion every 4.2 hours. Our orbital and spectroscopic analysis excludes a normal star, which would not fit within the orbit, as well as a second stripped star which would have been detectable in our spectra. Adding constraints from high cadence photometry gives a 99.4% probability that the companion is too massive to be a white dwarf, making a neutron star the most likely companion. We model the system's future and predict that the stripped star will transfer mass again before exploding as a hydrogen-free supernova, leaving a bound pair of neutron stars that will merge after 220 Myr. This system reveals a previously unconstrained phase of binary evolution, uncovering a new class of systems that probes the uncertain physics of massive stars, linking to observable gravitational-wave sources.