Gonzalo Morras, Geraint Pratten, Patricia Schmidt

We revisit the population properties of neutron star-black hole (NSBH) mergers using low-mass compact binary coalescences reported through GWTC-4. Employing pyEFPE, an inspiral-only waveform model that captures both orbital eccentricity and spin-induced precession, we reanalyse all binary neutron star (BNS) and NSBH events observed via gravitational waves. The BNS systems GW170817 and GW190425 are fully consistent with quasi-circular inspirals, while GW200105 stands out among the NSBH binaries as the only system exhibiting significant residual eccentricity at 20 Hz, strengthening evidence for dynamically driven formation pathways. The remaining NSBH events show no measurable eccentricity and appear broadly compatible with low-spin binaries formed through isolated stellar evolution. Using hierarchical Bayesian inference, we obtain the first joint constraints on the mass, spin, and eccentricity distributions of NSBH binaries. Our results also yield the first simultaneous constraints on spin precession and orbital eccentricity in NSBH mergers, while the inferred merger rates remain fully consistent with previous LVK measurements. Treating all NSBH systems as a single population yields results compatible with formation in hierarchical triples, whereas the quasi-circular population remains broadly consistent with isolated evolution. Our results highlight the emerging role of eccentricity as a key discriminator between formation channels. As the number of NSBH detections grows, joint constraints on masses, spins, and orbital eccentricity will enable increasingly sharp tests of dynamical versus isolated binary evolution, establishing NSBH systems as powerful probes of compact-object astrophysics.