  Recent experimental data for the complete wetting behavior of pure 4He and of
3He-4He mixtures exposed to solid substrates show that there is a change of the
corresponding film thicknesses L upon approaching thermodynamically the
lambda-transition and the tricritical end point, respectively, which can be
attributed to critical Casimir forces f_C. We calculate the scaling functions
vartheta of f_C within models representing the corresponding universality
classes. For the mixtures our analysis provides an understanding of the rich
behavior of vartheta deduced from the experimental data and predicts the
crossover behavior between the tricritical point and the lambda-transition of
pure 4He which are connected by a line of critical points. The formation of a
'soft-mode' phase within the wetting films gives rise to a pronounced maximum
of f_C below the tricritical point as observed experimentally. Near the
tricritical point we find logarithmic corrections ~L^(-3)(ln L)^(1/2) for the
leading behavior of vartheta dominating the contributions from the background
dispersion forces.
