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We present a systematic survey for mass-transferring and recently-detachedcataclysmic variables (CVs) with evolved secondaries, which are progenitors ofextremely low-mass white dwarfs (ELM WDs), AM CVn systems, and detachedultracompact binaries. We select targets below the main sequence in the Gaiacolor-magnitude diagram with ZTF light curves showing large-amplitudeellipsoidal variability and orbital period $P_{\rm orb} < 6$ hr. This yields 51candidates brighter than G=18, of which we have obtained many-epoch spectra for21. We confirm all 21 to be completely -- or nearly -- Roche lobe filling closebinaries. 13 show evidence of ongoing mass transfer, which has likely justceased in the other 8. Most of the secondaries are hotter than any previouslyknown CV donors, with temperatures $4700<T_{{\rm eff}}/{\rm K}<8000$.Remarkably, all secondaries with $T_{\rm eff} \gtrsim 7000\,\rm K$ appear to bedetached, while all cooler secondaries are still mass-transferring. Thistransition likely marks the temperature where magnetic braking becomesinefficient due to loss of the donor's convective envelope. Most of theproto-WD secondaries have masses near $0.15\,M_{\odot}$; their companions havemasses near $0.8\,M_{\odot}$. We infer a space density of $\sim 60\,\rmkpc^{-3}$, roughly 80 times lower than that of normal CVs and three times lowerthan that of ELM WDs. The implied Galactic birth rate, $\mathcal{R}\sim 60\,\rmMyr^{-1}$, is half that of AM CVn binaries. Most systems are well-described byMESA models for CVs in which mass transfer begins only as the donor leaves themain sequence. All are predicted to reach minimum periods $5\lesssim P_{\rmorb}/{\rm min}\lesssim30$ within a Hubble time, where they will become AM CVnbinaries or merge. This sample triples the known evolved CV population andoffers broad opportunities for improving understanding of the compact binarypopulation.

Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs