  We have detected asymmetry in the symbiotic star CH Cyg through the
measurement of precision closure-phase with the IONIC beam combiner, at the
IOTA interferometer. The position of the asymmetry changes with time and is
correlated with the phase of the 2.1-yr period found in the radial velocity
measurements for this star. We can model the time-dependent asymmetry either as
the orbit of a low-mass companion around the M giant or as an asymmetric, 20%
change in brightness across the M giant. We do not detect a change in the size
of the star during a 3 year monitoring period neither with respect to time nor
with respect to wavelength. We find a spherical dust-shell with an emission
size of 2.2+/-0.1 D* FWHM around the M giant star. The star to dust flux ratio
is estimated to be 11.63+/-0.3. While the most likely explanation for the 20%
change in brightness is non-radial pulsation we argue that a low-mass companion
in close orbit could be the physical cause of the pulsation. The combined
effect of pulsation and low-mass companion could explain the behaviour revealed
by the radial-velocity curves and the time-dependent asymmetry detected in the
closure-phase data. If CH Cyg is a typical long secondary period variable then
these variations could be explained by the effect of an orbiting low-mass
companion on the primary star.
