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ASASSN-V J205543.90+240033.5 (ASJ2055) is a possible post-common envelopebinary system. Its optical photometric data shows an orbital variation about$0.52$~days and a fast period modulation of $P_0\sim 9.77$~minute, whose originis unknown. In this {\it Letter}, we report an evidence of the stellaroscillation of the companion star as the origin of the fast period modulation.We analyze the photometric data taken by TESS, Liverpool telescope, and LulinOne-meter Telescope. It is found that the period of the 9.77-minute signalmeasured in 2022 August is significantly shorter than that in 2021 July/August,and the magnitude of the change is of the order of $|\triangle P_0|/P_0\sim0.0008(4)$. Such a large variation will be incompatible with the scenario ofthe white dwarf spin as the origin of the 9.77-minute periodic modulation. Wesuggest that the fast periodic signal is related to the emission from theirradiated companion star rather than that of the white dwarf. Using existingphotometric data covering a wide wavelength range, we estimate that the hotwhite dwarf in ASJ2055 has a temperature of $T_{eff}\sim 80000$~K and isheating the oscillating M-type main-sequence star with $T_{eff}\sim 3500$~K onits un-irradiated surface. The stellar oscillation of M-type main-sequence starhas been predicted in theoretical studies, but no observational confirmationhas been done. ASJ2055, therefore, has a potential to be a unique laboratory toinvestigate the stellar oscillation of a M-type main-sequence star and theheating effect on the stellar oscillation.

Evidence of stellar oscillations in the post-common envelop binary candidate ASASSN-V J205543.90+240033.5