We present the discovery of EVR-CB-004, a close binary with a remnant stellar core and an unseen white dwarf companion. The analysis in this work reveals the primary is potentially an inflated hot subdwarf (sdO) and more likely is a rarer post-blue horizontal branch (post-BHB) star. Post-BHBs are the short-lived shell-burning final stage of a blue horizontal star or hot subdwarf before transitioning to a WD. This object was discovered using Evryscope photometric data in a southern-all-sky hot subdwarf variability survey. The photometric light curve for EVR-CB-004 shows multi-component variability from ellipsoidal deformation of the primary and from Doppler boosting as well as gravitational limb darkening. EVR-CB-004 is one of just a handful of known systems, and has a long period (6.08426 hours) and large amplitude ellipsoidal modulation (16.0 $\%$ change in brightness from maximum to minimum) for these extremely close binary systems, while the properties of the primary make it a truly unique system. EVR-CB-004 also shows a peculiar low-amplitude (less than $1\%$) sinusoidal light curve variation with a period that is a 1/3 resonance of the binary period. We tentatively identify this additional variation source as a tidally-induced resonant pulsation, and we suggest followup observations that could verify this interpretation. From the evolutionary state of the system, its components, and its mass fraction, EVR-CB-004 is a strong merger candidate to form a single high-mass ($\approx1.2M_{\odot}$) WD. EVR-CB-004 offers a glimpse into a brief phase of a remnant core evolution and secondary variation, not seen before in a compact binary.

中文翻译：

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EVR-CB-004：Evryscope 发现的紧凑双星中膨胀的热亚矮 O 星 + 看不见的 WD 伴星

我们展示了 EVR-CB-004 的发现，这是一个具有残余恒星核心和一个看不见的白矮星伴星的紧密双星。这项工作中的分析表明，主星可能是一颗膨胀的热亚矮星（sdO），更有可能是一颗更罕见的后蓝色水平分支（后 BHB）恒星。Post-BHBs 是蓝色水平星或热亚矮星在过渡到 WD 之前短暂的壳燃烧最后阶段。该天体是使用 Evryscope 光度数据在南方全天热亚矮变率调查中发现的。EVR-CB-004 的测光光度曲线显示了由初级椭圆形变形和多普勒增强以及重力临边变暗引起的多分量可变性。EVR-CB-004 是为数不多的已知系统之一，它具有很长的周期（6.08426 小时）和大振幅椭球调制（16.0 小时）。0 $\%$ 亮度从最大到最小的变化）对于这些非常接近的双星系统，而主要的属性使它成为一个真正独特的系统。EVR-CB-004 还显示出一种特殊的低振幅（小于 $1\%$）正弦光曲线变化，其周期为二进制周期的 1/3 共振。我们暂时将这个额外的变化源确定为潮汐引起的共振脉动，我们建议后续观察可以验证这种解释。从系统的演化状态、它的组成部分和它的质量分数来看，EVR-CB-004 是一个强大的合并候选者，可以形成一个单一的大质量（$\approx1.2M_{\odot}$）WD。EVR-CB-004 提供了对残余核心演化和次生变异的简短阶段的一瞥，这是以前在紧凑的双星中没有看到的。