GK Cep is an eclipsing binary that contains two A-type components with an orbital period of 0.936 d. The high mass ratio and the contact or near-contact configuration make it a key target to understand binary evolution. However, the physical properties and evolutionary state are still unknown. Here we present the UV light curve obtained by using the lunar-based ultraviolet telescope (LUT) and several eclipse times obtained with the Sino-Thai 70-cm telescope and the 1.0-m telescope at Yunnan Observatories. Analysis of the LUT light curve suggests that GK Cep is a near-contact semidetached system where the slightly less massive component is the hotter one that is filling the critical Roche lobe. Meanwhile, a hotspot is found on the neck of the more massive but cooler component. By using all available times for the light minimum, it is detected that the O–C curve shows a cyclic oscillation while it undergoes an upward parabolic change. The increase of the orbital period, the high mass ratio, the semidetached configuration with a lobe-filling less massive component, and the presence of a hotspot on the more massive component all suggest that GK Cep is in a key evolutionary state just after the shortest-period evolutionary stage during the rapid mass transfer. The cyclic change in the O–C curve reveals that GK Cep contains a solar-type stellar companion in a triply hierarchical system that is in agreement with the spectroscopic result. The third body contributes about 2.6 per cent to the total light of the triple system.

中文翻译：

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分级三重系统 GK Cep 中双组分之间的快速传质

GK Cep 是一个食双星，包含两个 A 型分量，轨道周期为 0.936 d。高质量比和接触或近接触配置使其成为理解二元进化的关键目标。然而，物理特性和进化状态仍然未知。在这里，我们展示了使用月基紫外望远镜（LUT）获得的紫外光曲线以及云南天文台中泰70厘米望远镜和1.0米望远镜获得的几个日食时间。LUT 光曲线分析表明，GK Cep 是一种近接触半分离系统，其中质量稍小的成分是填充关键 Roche 瓣的较热成分。与此同时，在更大但更冷的部件的颈部发现了一个热点。通过将所有可用时间用于光最小值，检测到O-C曲线呈现周期性振荡，同时经历向上的抛物线变化。轨道周期的增加、质量比的高、半分离构型中填充的质量较小的成分以及较大质量成分上热点的存在都表明 GK Cep 处于最短进化状态之后的关键进化状态。 - 快速传质期间的演化阶段。O-C 曲线的循环变化表明，GK Cep 包含一个太阳型恒星伴星，处于与光谱结果一致的三级系统中。第三个身体对三重系统的总光贡献约 2.6%。具有填充较小质量组分的半分离构型以及在较大质量组分上存在热点都表明GK Cep在快速传质期间的最短演化阶段之后处于关键演化状态。O-C 曲线的循环变化表明，GK Cep 包含一个太阳型恒星伴星，处于与光谱结果一致的三级系统中。第三个身体对三重系统的总光贡献约 2.6%。具有填充较小质量组分的半分离构型以及在较大质量组分上存在热点都表明GK Cep在快速传质期间的最短演化阶段之后处于关键演化状态。O-C 曲线的循环变化表明，GK Cep 包含一个太阳型恒星伴星，处于与光谱结果一致的三级系统中。第三个身体对三重系统的总光贡献约 2.6%。O-C 曲线的循环变化表明，GK Cep 包含一个太阳型恒星伴星，处于与光谱结果一致的三级系统中。第三个身体对三重系统的总光贡献约 2.6%。O-C 曲线的循环变化表明，GK Cep 包含一个太阳型恒星伴星，处于与光谱结果一致的三级系统中。第三个身体对三重系统的总光贡献约 2.6%。