The silicate carbon star V778 Cyg is a source of 22 GHz water maser emission which was recently resolved by MERLIN .
Observations revealed an elongated \cal { S } -like structure along which the velocities of the maser features show a linear dependence on the impact parameter .
This is consistent with a doubly-warped m = 2 disc observed edge-on .
Water masers and silicate dust emission ( detected by IRAS and ISO ) have a common origin in O-rich material and are likely to be co-located in the disc .
We propose a detailed self-consistent model of a masing gas-dust disc around a companion to the carbon star in a binary system , which allows us to estimate the companion mass of 1.7 \pm 0.1 M _ { \odot } , the disc radius of 40 \pm 3 AU and the distance between companions of \sim 80 AU .
Using a dust-gas coupling model for water masing , we calculate the maser power self-consistently , accounting for both the gas and the dust energy balances .
Comparing the simulation results with the observational data , we deduce the main physical parameters of the masing disc , such as the gas and dust temperatures and their densities .
We also present an analysis of the stability of the disc .