  We present multi-band photometric and spectroscopic observations of the type
II supernova, (SN) 2019va, which shows an unusually flat plateau-phase
evolution in its V-band light curve. Its pseudo-bolometric light curve even
shows a weak brightening towards the end of the plateau phase. These uncommon
features are related to the influence of 56Ni decay on the light curve during
the plateau phase, when the SN emission is usually dominated by cooling of the
envelope. The inferred 56Ni mass of SN 2019va is 0.088+/-0.018 solar mass,
which is significantly larger than most SNe II. To estimate the influence of
56Ni decay on the plateau-phase light curve, we calculate the ratio (dubbed as
eta_Ni) between the integrated time-weighted energy from 56Ni decay and that
from envelope cooling within the plateau phase, obtaining a value of 0.8 for SN
2019va, which is the second largest value among SNe II that have been measured.
After removing the influence of 56Ni decay on the plateau-phase light curve, we
found that the progenitor/explosion parameters derived for SN 2019va are more
reasonable. In addition, SN 2019va is found to have weaker metal lines in its
spectra compared to other SNe IIP at similar epochs, implying a low-metallicity
progenitor, which is consistent with the metal-poor environment inferred from
the host-galaxy spectrum. We further discuss the possible reasons that might
lead to SN 2019va-like events.
