text: 233,90,1425394215958241285,952949678533849088,Kareem El-Badry,"New paper! We present first results from a survey of compact binary stars with ongoing and just-terminated mass transfer. 1/ <LINK> <LINK> We select targets from below the main-sequence in the #GaiaMission color-magnitude diagram that have @ztfsurvey light curves dominated by ellipsoidal variability (due to tidal deformation). 2/ <LINK> This selects objects that (a) are hotter and smaller than normal main-sequence stars, and (b) are dominated a star (the ""donor""), not an accretion disk. We vet targets with spectroscopic follow-up. 3/ <LINK> Our final sample of objects lives in a previously (almost) empty region of the HR diagram, between extremely low-mass white dwarfs and main-sequence stars (and ""normal"" cataclysmic variable donors). 4/ <LINK> Most of the objects we find are hotter than any previously identified similar objects. We think this is because they have undergone more nuclear evolution. That is,... 5/ <LINK> ... these objects form when mass is stripped off the outside of a star by a white dwarf companion. We think mass transfer began late in these objects, after a helium core had started to form. Today, just the helium core and a thin envelope are left. 6/ All the hottest (Teff &gt;~ 7000 K) objects appear to have just ended mass transfer, while the cooler ones still have ongoing mass transfer. We think this means magnetic braking becomes inefficient at Teff &gt;~ 7000 K, when stars lose their convective envelopes, slowing inspiral. 7/ This systematic survey allows us to derive a space density and birth rate for these evolved-CVs-turning-into-extremely-low-mass-white-dwarfs. Our inferred birth rate is about half that of the birth rate of ultracompact mass-transferring ""AM-CVn"" binaries. 8/ <LINK> We think many of these objects will turn into AM-CVn binaries within a few Gyr. The birth rate suggests this channel may be (is likely to be?) one of the dominant formation channels for AM-CVn binaries. We have more observations and analysis of these objects in the works! 9/9.",https://arxiv.org/abs/2108.04255,"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 donor's 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 $5\lesssim P_{\rm orb}/{\rm min}\lesssim30$ 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. ","Birth of the ELMs: a ZTF survey for evolved cataclysmic variables