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tokens: the two objects could also exhaust helium shell burning and simply cool into double degenerates. Upon merger, they would likely form a rapidly rotating CO WD, potentially preceded by an R CrB phase if substantial He remains, as expected for hybrid WDs (Perets et al. 2019 ). For further details on these systems, please see Kupfer et al. ( 2020a , 2020b ). 6. Conclusion Using data from the ZTF, we have significantly increased the number of known binary systems with an orbital period of less than one hour. Using high-speed photometric follow-up in combination with spectroscopic follow-up, we have characterized the physical parameters of the systems using model-dependent techniques invoking WD mass–radius relations. In future work, we hope to obtain more model-independent measurements such as orbital decay rates to more robustly characterize the physical parameters of these systems. We will continue to analyze ZTF data as it accumulates more epochs and discovers more such objects. At the current stage of the survey, the sample we have discovered exhibits a remarkably high fraction of pairs of He WDs, likely due to selection effects. As we continue the survey, we anticipate discovering more systems and that our discoveries will begin to include other classes of sources, such as pairs of CO WDs. All of the detached systems in our sample should be undergoing rapid orbital decay due to general relativity, and we expect to detect this orbital decay in every detached system with high significance before LISA launches. The algorithms and analysis techniques we are developing will be applicable to upcoming surveys such as the one that will be conducted by the Vera Rubin Observatory (LSST Science Collaboration et al. 2009 ). These data sets will be accompanied by their own challenges (in particular, lower cadence, split across many filters). Given the clear promise demonstrated by the discoveries emerging from ZTF, we are optimistic that optical time-domain surveys will continue to revolutionize our understanding of binaries in the millihertz regime. These surveys will set the stage for LISA, having discovered dozens, if not hundreds, of sources by the time the instrument begins to operate (Korol et al. 2017 ). In future work, we will characterize our recovery efficiency in greater detail using synthetic lightcurves, in order to better characterize ZTF’s sensitivity to detecting the population of DWDs in the galaxy. Efforts are currently underway to increase sensitivity to the shortest-period binary systems by accounting for their orbital period evolution, which requires algorithms that can accommodate acceleration searches (Katz et al. 2020 ). Binaries with compact object components are transitioning into a golden age, as we enter an era when time-domain surveys such as ZTF enable detection via periodic features. Massively multiplexed spectroscopic surveys acquiring multiple epochs such as SDSS-V (Kollmeier et al. 2017 ) and LAMOST (Cui et al. 2012 )