LAMOST J0140355 + 392651: an evolved cataclysmic variable donor transitioning to become an extremely low-mass white dwarf

We present LAMOST J0140355 + 392651 (hereafter J0140), a close (P_orb = 3.81 h) binary containing a bloated, low-mass ($M \approx 0.15 \, \mathrm{M}_{\odot }$) proto-white dwarf (WD) and a massive ($M \approx 0.95\, \mathrm{M}_{\odot }$) 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_eff = 6800 ± 100 K) and more puffy ($\log \left[g/\left({\rm cm\, s^{-2}}\right)\right]=4.74\pm 0.07$) 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 ($R/R_{{\rm Roche\, lobe}}=0.99\pm 0.01$), 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 α emission, perhaps due to accretion on to 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 towards 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_orb ≲ 10 min 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.