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\,hr. 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 and spectroscopic modeling confirm its nature, yielding an estimated inclination of $i = 86.2^{+0.6}_{-0.2}\,\rm degrees$ and primary and secondary effective temperatures of $\textrm{T}_\textrm{eff} = 28,000^{+500}_{-500}\,K$ and $\textrm{T}_\textrm{eff} = 17,600^{+400}_{-400}\,K$ respectively. This system adds to a growing list of sources for future gravitational-wave detectors and contributes to the demographic analysis of double degenerates.