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We present a systematic survey for mass-transferring and recently-detached cataclysmic variables (CVs) with evolved secondaries, which are progenitors of extremely low-mass white dwarfs (ELM WDs), AM CVn systems, and detached ultracompact binaries. We select targets below the main sequence in the Gaia color-magnitude diagram with ZTF light curves showing large-amplitude ellipsoidal variability and orbital period $P_{rm orb} < 6$ hr. This yields 51 candidates brighter than G=18, of which we have obtained many-epoch spectra for 21. We confirm all 21 to be completely -- or nearly -- Roche lobe filling close binaries. 13 show evidence of ongoing mass transfer, which has likely just ceased in the other 8. Most of the secondaries are hotter than any previously known CV donors, with temperatures $4700<T_{{rm eff}}/{rm K}<8000$. Remarkably, all secondaries with $T_{rm eff} gtrsim 7000,rm K$ appear to be detached, while all cooler secondaries are still mass-transferring. This transition likely marks the temperature where magnetic braking becomes inefficient due to loss of the donors convective envelope. Most of the proto-WD secondaries have masses near $0.15,M_{odot}$; their companions have masses near $0.8,M_{odot}$. We infer a space density of $sim 60,rm kpc^{-3}$, roughly 80 times lower than that of normal CVs and three times lower than that of ELM WDs. The implied Galactic birth rate, $mathcal{R}sim 60,rm Myr^{-1}$, is half that of AM CVn binaries. Most systems are well-described by MESA models for CVs in which mass transfer begins only as the donor leaves the main sequence. All are predicted to reach minimum periods $5lesssim P_{rm orb}/{rm min}lesssim30$ within a Hubble time, where they will become AM CVn binaries or merge. This sample triples the known evolved CV population and offers broad opportunities for improving understanding of the compact binary population.
Extremely low mass (ELM) white dwarfs (WDs) with masses <0.25 Msun are rare objects that result from compact binary evolution. Here, we present a targeted spectroscopic survey of ELM WD candidates selected by color. The survey is 71% complete and has
We begin the search for extremely-low mass ($Mleq0.3M_{odot}$, ELM) white dwarfs (WDs) in the southern sky based on photometry from the VST ATLAS and SkyMapper surveys. We use a similar color-selection method as the Hypervelocity star survey. We swit
The origin of magnetic fields in isolated and binary white dwarfs has been investigated in a series of recent papers. One proposal is that magnetic fields are generated through an alpha-omega dynamo during common envelope evolution. Here we present p
We present LAMOST J0140355+392651 (hereafter J0140), a close ($P_{rm orb} = 3.81$ hours) binary containing a bloated, low-mass ($M approx 0.15 M_{odot}$) proto-white dwarf (WD) and a massive ($Mapprox 0.95,M_{odot}$) WD companion. The systems optical
Extremely low-mass white dwarfs (ELM WDs) are helium WDs with a mass less than $sim$$0.3rm;M_odot$. Most ELM WDs are found in double degenerates (DDs) in the ELM Survey led by Brown and Kilic. These systems are supposed to be significant gravitationa