  CH Cygni began producing rapid, stochastic optical variations with the onset
of symbiotic activity in 1963. We use changes in this flickering between 1997
and 2000 to diagnose the state of the accretion disk during this time. In the
1998 high state, the luminosity of the B-band flickering component was
typically more than 20 times higher than in the 1997 and 2000 low states.
Therefore, the physical process or region that produces the flickering was also
primarily responsible for the large optical flux increase in the 1998 high
state. Assuming that the rapid, stochastic optical variations in CH Cygni come
from the accretion disk, as in cataclysmic variable stars, a change in the
accretion rate through the disk led to the 1998 bright state. All flickering
disappeared in 1999, when the accreting WD was eclipsed by the red giant
orbiting with a period of approximately 14 yr, according to the ephemeris of
Hinkle et al. and the interpretation of Eyres et al. We did not find any
evidence for periodic or quasi-periodic oscillations in the optical emission
from CH Cygni in either the high or low state, and we discuss the implications
for magnetic propeller models of this system. As one alternative to propeller
models, we propose that the activity in CH Cygni is driven by accretion through
a disk with a thermal-viscous instability, similar to the instabilities
believed to exist in dwarf novae and suggested for FU Ori pre-main-sequence
stars and soft X-ray transients.
