ZTF J1901+5309: a 40.6-min orbital period eclipsing double white dwarf system

The Zwicky Transient Facility has begun to discover binary systems with orbital periods that are less than 1 h. Combined with dedicated follow-up systems, which allow for high-cadence photometry of these sources, systematic confirmation and characterization of these sources are now possible. Here, we report the discovery of ZTF J190125.42+530929.5, a 40.6-min orbital period, eclipsing double white dwarf binary. Both photometric modelling and spectroscopic modelling confirm its nature, yielding an estimated inclination of $i = 86.2^{+0.6}_{-0.2}\, \rm deg$ and primary and secondary effective temperatures of $\textrm{{T}}_\textrm{eff} = 28\,000^{+500}_{-500}$ and $17\,600^{+400}_{-400}\, \mathrm{ K}$, respectively. This system adds to a growing list of sources for future gravitational-wave detectors and contributes to the demographic analysis of double degenerates.