Nine new eclipsing accreting ultracompact white dwarf binaries from the Zwicky Transient Facility

Abstract

Accreting ultracompact white dwarf binaries (accreting UCBs) transfer helium-rich material at orbital periods below about 65 min, and mainly divide into hydrogen-free AM CVn binaries and helium cataclysmic variables. Eclipsing systems are of particular value: they allow the component masses and radii to be measured independently of any model, the most direct test of the proposed formation channels, and they form a sample with a simple, well-understood selection function. Only 21 eclipsing accreting white dwarf UCBs have been published. We aim to enlarge the sample of eclipsing accreting UCBs available for detailed characterisation. We searched the Zwicky Transient Facility (ZTF) light curves of Gaia-selected candidates and of ZTF-selected dwarf novae for short-period eclipse signals using a box least-squares period search. Identification spectra of nine systems were obtained with LRIS on the Keck I telescope. We present nine new discoveries and re-analyse the four systems first reported by Khalil et al. (2024). Eclipses are detected directly in the survey photometry of all 13 systems, which have orbital periods between 26.0 and 65.0 min and median ZTF magnitudes $g = 18.7-21.5$. Ten are consistent with being AM CVn binaries, and two are likely helium cataclysmic variables (of which one may host a high-state disc). The remaining system, ZTF J2116+2446, shows only hydrogen lines, making it the shortest-period UCB with hydrogen but no detected helium; it may have evolved from a white dwarf-brown dwarf binary. The nine new systems increase the number of published eclipsing accreting UCBs from 21 to 30. Because they are eclipsing, they are immediately available for the high-speed photometric modelling that yields model-independent masses and radii, to be presented in future work, including for the three hydrogen-rich systems that are key tests of the evolved cataclysmic-variable channel.