Here we report observations of ZTF J1813+4251, a 51-minute-orbital-period, fully eclipsing binary system consisting of a star with a temperature comparable to that of the Sun but a density 100 times greater owing to its helium-rich composition, accreting onto a white dwarf. However, no known transitional CV is expected to reach an orbital period short enough to account for most of the helium CV population, leaving the role of this evolutionary pathway unclear. These transitional CVs have been proposed as progenitors of helium CVs 13, 14, 15, 16, 17, 18. One way to achieve these short periods requires the donor star to have undergone substantial nuclear evolution before interacting with the white dwarf 10, 11, 12, 13, 14, and it is expected that these objects will transition to helium accretion. Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes 1, 2, 3, 4, 5, 6, 7, 8, 9.
A dense 0.1-solar-mass star in a 51-minute-orbital-period eclipsing binary
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