We present LAMOST J0140355 + 392651 (hereafter J0140), a close (Porb = 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 (Teff = 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 Porb ≲ 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.

中文翻译：

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LAMOST J0140355 + 392651：进化的灾难性可变供体转变为质量极低的白矮星

我们提出 LAMOST J0140355 + 392651（以下简称 J0140），一个封闭的 (Porb = 3.81 h) 二进制文件，包含一个臃肿的、低质量的 ($M \approx 0.15 \, \mathrm{M}_{\odot }$) 原始白色矮星 (WD) 和一个巨大的 ($M \approx 0.95\, \mathrm{M}_{\odot }$) WD 伴星。该系统的光学光变曲线主要由大振幅椭球变化控制，但也表现出额外的散射，可能是由脉动驱动的。原型 WD 更冷 (Teff = 6800 ± 100 K) 且更蓬松 ($\log \left[g/\left({\rm cm\, s^{-2}}\right)\right]=4.74 \pm 0.07$) 比任何已知的极低质量 (ELM) WD，但比任何已知的灾难变量 (CV) 供体更热。它完全或几乎充满了它的罗氏叶 ($R/R_{{\rm Roche\, lobe}}=0.99\pm 0.01$)，表明正在进行或最近终止的传质。没有观察到类似矮新星的爆发。光谱以原始 WD 为主，但显示出 H α 发射的初步迹象，可能是由于对大质量 WD 的吸积。该系统的特性与 CV 的台面二元演化模型与供体在传质开始之前经历了显着的核演化非常匹配。在这些模型中，臃肿的 proto-WD 要么仍然通过稳定的罗氏叶溢出而失去质量，要么直到最近才这样做。在任何一种情况下，它都在以接近恒定的光度向更高的温度发展，成为 ELM WD。如果系统被分离，当供体变得太热以至于磁制动无法保持有效时，传质可能会结束。进化模型预测二进制将在几个 Gyr 内缩小到 Porb ≲ 10 分钟，届时它将合并或成为 AM CVn 二进制。