EM Cyg is an eclipsing dwarf nova with an orbital period above the period gap of cataclysmic variables. Based on the whole outburst of EM Cyg observed by the Transiting Exoplanet Survey Satellite (TESS) from 2019 July 18–August 14, the evolution of quasi-periodic oscillations (QPOs) in EM Cyg was investigated. A QPO with a period of about 1000 s was found during both outburst and quiescence. In addition, we found another QPO with a period of about 500 s, which was present during the outburst but disappeared as the outburst ended. QPOs should be caused by the activity of the accretion disc. Using the data obtained during the outburst, we analysed the characteristics of the light curves and found that the eclipsing depth increased and the light-minimum moment earlier during the outburst. In addition, with the new eclipse times obtained by TESS and from our observations, an O − C diagram for this system was produced. According to the O − C analysis, the orbital period is increasing at a rate of $\dot{P} = 1.66(\pm 0.36) \times 10^{-8}\, \mathrm{d\,yr^{-1}}$. There is a cyclic variation in the O − C curve that might be caused by the light-traveltime effect. A third body may exist in the EM Cyg system with the mass $m_3\sin {i{^{\prime }}} = 0.097(\pm 0.000\,35) \, \mathrm{M}_{\odot }$.

中文翻译：

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食矮新星EM Cyg的准周期振荡和长期轨道周期变化

EM Cyg 是一颗食矮新星，其轨道周期高于灾变变量的周期间隙。基于凌日系外行星调查卫星 (TESS) 于 2019 年 7 月 18 日至 8 月 14 日观测到的整个 EM Cyg 爆发，研究了 EM Cyg 中准周期振荡 (QPO) 的演变。在爆发和静止期间都发现了一个周期约为 1000 s 的 QPO。此外，我们还发现了另一个周期约为 500 s 的 QPO，它在爆发期间存在，但随着爆发结束而消失。QPO 应该是由吸积盘的活动引起的。利用爆发期间获得的数据，我们分析了光变曲线的特征，发现在爆发期间食深增加，光最小时刻提前。此外，通过 TESS 获得的新日食时间和我们的观察，生成了该系统的 O-C 图。根据 O − C 分析，轨道周期以 $\dot{P} = 1.66(\pm 0.36) \times 10^{-8}\, \mathrm{d\,yr^{- 1}}$。O - C 曲线有一个循环变化，这可能是由光传播时间效应引起的。第三个物体可能存在于 EM Cyg 系统中，其质量为 $m_3\sin {i{^{\prime }}} = 0.097(\pm 0.000\,35) \, \mathrm{M}_{\odot }$ .