An eclipsing 8.56 minutes orbital period mass-transferring binary Chickles, Emma T. Chakraborty, Joheen Burdge, Kevin B. Dhillon, Vik S. Draghis, Paul El-Badry, Kareem Green, Matthew J. Householder, Aaron Hughes, Sarah Layden, Christopher Littlefair, Stuart P. Munday, James Pelisoli, Ingrid Redden, Maya S. Tonry, John van Roestel, Joannes C Angile, Francesco Elio Brown, Alex J. Segura, Noel Castro Dinsmore, Jack Dyer, Martin Furesz, Gabor Gabutti, Michelle Garbutt, James García-Mejía, Juliana Jarvis, Daniel Kennedy, Mark R. Kerry, Paul Mccormac, James Mo, Geoffrey Osip, Dave Parsons, Steven Pike, Eleanor Piotrowski, John J. Romani, Roger W. Sahman, David Simcoe, Rob ddc:520 We report the discovery of ATLAS J101342.5−451656.8 (hereafter ATLAS J1013−4516), an 8.56 minute orbital-period mass-transferring AM Canum Venaticorum (AM CVn) binary with a mean Gaia magnitude of G = 19.51, identified via periodic variability in light curves from the Asteroid Terrestrial-impact Last Alert System (ATLAS) of Gaia white dwarf candidates. Follow-up with the Large Lenslet Array Magellan Spectrograph shows a helium-dominated accretion disk, and high-speed ULTRACAM photometry reveals pronounced primary and secondary eclipses. We construct a decade-long timing baseline leveraging light curves from the ATLAS and Gaia surveys, as well as the high-speed imagers ULTRACAM on the New Energy Telescope and proto-Lightspeed on the Magellan Clay telescope. From this timing baseline, we measure an orbital period derivative of P 1.60 0.07 10 = ± × 12 s s−1. Interpreted in the context of stable mass transfer, the magnitude and sign of P indicate that the orbital evolution is governed by the interplay between gravitationalwave-driven angular-momentum losses and mass transfer, directly probing the donor’s structural response to mass loss. We constrain the accretor and donor mass based on stable mass-transfer arguments assuming angularmomentum loss dominated by gravitational-wave emission, allowing us to infer the characteristic gravitational wave strain of the binary for future space-based GW observatories such as the Laser Interferometer Space Antenna (LISA). We predict a characteristic strain corresponding to a 4 yr LISA signal-to-noise ratio ≳10, establishing ATLAS J1013−4516 as a strong prospective LISA source that will probe long-term orbital evolution in the mass-transferring regime. IOP Publishing 2026 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/21705 https://research-explorer.ista.ac.at/download/21705/21782 Chickles ET, Chakraborty J, Burdge KB, et al. An eclipsing 8.56 minutes orbital period mass-transferring binary. <i>The Astrophysical Journal</i>. 2026;1000(2). doi:<a href="https://doi.org/10.3847/1538-4357/ae4871">10.3847/1538-4357/ae4871</a> eng info:eu-repo/semantics/altIdentifier/doi/10.3847/1538-4357/ae4871 info:eu-repo/semantics/altIdentifier/issn/0004-637X info:eu-repo/semantics/altIdentifier/e-issn/1538-4357 info:eu-repo/semantics/altIdentifier/arxiv/2601.07925 info:eu-repo/semantics/openAccess