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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 $m_3\sin {i{^{\prime }}} = 0.097(\pm 0.000\,35) \, \mathrm{M}_{\odot }$.

Quasi-periodic oscillations and long-term orbital period variation of the eclipsing dwarf nova EM Cyg