  ASASSN-V J205543.90+240033.5 (ASJ2055) is a possible post-common envelope
binary 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 origin
is unknown. In this {\it Letter}, we report an evidence of the stellar
oscillation of the companion star as the origin of the fast period modulation.
We analyze the photometric data taken by TESS, Liverpool telescope, and Lulin
One-meter Telescope. It is found that the period of the 9.77-minute signal
measured in 2022 August is significantly shorter than that in 2021 July/August,
and the magnitude of the change is of the order of $|\triangle P_0|/P_0\sim
0.0008(4)$. Such a large variation will be incompatible with the scenario of
the white dwarf spin as the origin of the 9.77-minute periodic modulation. We
suggest that the fast periodic signal is related to the emission from the
irradiated companion star rather than that of the white dwarf. Using existing
photometric data covering a wide wavelength range, we estimate that the hot
white dwarf in ASJ2055 has a temperature of $T_{eff}\sim 80000$~K and is
heating the oscillating M-type main-sequence star with $T_{eff}\sim 3500$~K on
its un-irradiated surface. The stellar oscillation of M-type main-sequence star
has been predicted in theoretical studies, but no observational confirmation
has been done. ASJ2055, therefore, has a potential to be a unique laboratory to
investigate the stellar oscillation of a M-type main-sequence star and the
heating effect on the stellar oscillation.
