  Various scenarios have been proposed to explain the presence of silicate
features associated with carbon stars, such as V778 Cyg. We have attempted to
constrain these theories by means of mapping water maser mission from V778 Cyg.
The 22 GHz water maser emission from this star has been mapped using MERLIN
with an astrometric accuracy of 25 mas. The spatially- and
kinematically-resolved maser complex is displaced by about 190 mas from the
position of the C-star as measured 10 years earlier using Tycho. Our
simulations and analysis of available data show that this position difference
is unlikely to be due to proper motion if V778 Cyg is at the assumed distance
of 1.4 kpc. The maser components seem to form a distorted S-shaped structure
extended over 18 mas with a clear velocity gradient. We propose a model which
explains the observed water maser structure as an O-rich warped disk around a
companion of the C-star in V 778 binary system, which is seen almost edge-on.
Analysis of observational data, especially those obtained with MERLIN, suggests
that V778 Cyg (and, by implication, other silicate carbon stars) are binary
systems composed of a C-rich star and a companion which stores circumstellar
O-rich material.
