text: 484,70,1383054369097011204,952949678533849088,Kareem El-Badry,"New paper. We found a new (sort of) type of interacting binary star! <LINK> <LINK> It's a white dwarf and a tidally distorted, bloated, stripped helium core. It looks like a cataclysmic variable (white dwarf accreting from a normal star), but the donor star is much hotter than any known cataclysmic variable donor. <LINK> Conversely, it's cooler, more bloated, and more tidally distorted than known low-mass white dwarfs. <LINK> There are a few other known stars with similar temperature and luminosity (“sdA” stars) but they are not close being mass transferring and are mostly in wider binaries. We think this binary was a cataclysmic variable with a donor that started transferring mass to its white dwarf companion *just* at the end of the main sequence. Now it's becoming an extremely low-mass white dwarf. <LINK> In a few Gyr, the binary will shrink to extremely short periods, where it appear as an ultracompact white dwarf binary or maybe an ""AM CVn"" (ultra short period, mass-transferring) system. <LINK> There are more of these transitional systems to be characterized! We're looking forward to mapping the population.  thanks to @kenjshen, @thomkupfer, @bigticketdw, and other coauthors not on twitter. @PNeunteufel @kenjshen In the best-fit models, there is still a ~0.005 Msun H-burning envelope (which is why the contraction is slow). There are no shell flashes for the best-fit mass of 0.15 Msun, but there would be for &gt;0.18 Msun or so. Thanks!",https://arxiv.org/abs/2104.07033,"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 ($M\approx 0.95\,M_{\odot}$) WD companion. The system's optical light curve is dominated by large-amplitude ellipsoidal variability but also exhibits additional scatter, likely driven by pulsations. The proto-WD is cooler ($T_{\rm eff} = 6800\pm 100$ K) and more puffy ($\log\left[g/\left({\rm cm\,s^{-2}}\right)\right]=4.74\pm0.07$) than any known extremely low mass (ELM) WD, but hotter than any known cataclysmic variable (CV) donor. It either completely or very nearly fills its Roche lobe ($R/R_{{\rm Roche\,lobe}}=0.99\pm0.01$), suggesting ongoing or recently terminated mass transfer. No dwarf nova-like outbursts have been observed. The spectrum is dominated by the proto-WD but shows tentative hints of H$\alpha$ emission, perhaps due to accretion onto the massive WD. The properties of the system are well-matched by MESA binary evolution models of CVs with donors that underwent significant nuclear evolution before the onset of mass transfer. In these models, the bloated proto-WD is either still losing mass via stable Roche lobe overflow or was doing so until very recently. In either case, it is evolving toward higher temperatures at near-constant luminosity to become an ELM WD. If the system is detached, mass transfer likely ended when the donor became too hot for magnetic braking to remain efficient. Evolutionary models predict that the binary will shrink to $P_{\rm orb}\lesssim 10$ minutes within a few Gyr, when it will either merge or become an AM CVn binary. J0140 provides an observational link between the formation channels of CVs, ELM WDs, detached ultracompact WD binaries, and AM CVn systems. ","LAMOST J0140355+392651: An evolved cataclysmic variable donor