  Tricritical Casimir forces in $^3\text{He}$ -$^4\text{He}$ wetting films are
studied, within mean field theory, in therms of a suitable lattice gas model
for binary liquid mixtures with short--ranged surface fields. The proposed
model takes into account the continuous rotational symmetry O(2) of the
superfluid degrees of freedom associated with $^4\text{He}$ and it allows,
inter alia, for the occurrence of a vapor phase. As a result, the model
facilitates the formation of wetting films, which provides a strengthened
theoretical framework to describe available experimental data for tricritical
Casimir forces acting in $^3\text{He}$ -$^4\text{He}$ wetting films.
