Surprising Variability of the Planetary Nebula IC 4997 = QV Sge
  We present the results of a new epoch of a long-term photometric monitoring
of the variable planetary nebula IC~4997. The integral $UBV$ light curves
display a continuing brightening of 0.$^{m}15$ in $V$, a slight rise
(<0.$^{m}1$) in $B$, and constancy in $U$. The $B-V$ color has got redder from
0.$^{m}4$ in 2000 to 0.$^{m}7$ in 2019, whereas the $U-B$ color has not changed
significantly. New $JHKL$ photometry was obtained in 2019. We found the source
to be fainter by 0.$^{m}4$ in $L$ and bluer by 0.$^{m}2$ in the $K-L$ color if
compared to the data of 1999-2006. The long-term brightness variations are due
mostly to the changing input of emission lines to the integral light.
Low-dispersion spectroscopic observations carried out in 2010-2019 revealed a
continuing decrease in the $\lambda4363/\lambda4340$ ratio: it decreased by a
factor of $\sim3$ in 30 years and reached the level of 1960-1970. The absolute
intensities of [OIII] nebualr lines increased by a factor of $>2$ from 1990 to
2019, whereas the [OIII] $\lambda$4363 line had weakened by a factor of 2
comparing to the maximum value observed in 2000. The variation of H$\beta$
absolute intensity in 1960-2019 was shown to be similar to that of [OIII]
nebular lines, but of smaller amplitude. The electron density in the outer part
of the nebula was estimated from the [SII] and [ClIII] lines. Basing on the
data on absolute intensities we propose a possible scenario describing the
change of $N_e,T_e$ in 1970-2019. The spectral variability of IC~4997 could be
explained by a variation of electron temperature in the nebula caused by not so
much the change in ionizing flux from the central star as the variable stellar
wind and related processes. The photometric and spectral changes observed for
IC~4997 in 1960-2019 may be interpreted as an observable consequence of a
single episode of enhanced mass loss from the variable central star.
