  Four different types of free energies are computed by both thermodynamical
Bethe Ansatz (TBA) techniques and by weak coupling perturbation theory in an
integrable one-parameter deformation of the O(4) principal chiral sigma-model
(with SU(2)xU(1) symmetry). The model exhibits both `fermionic' and `bosonic'
type free energies and in all cases the perturbative and the TBA results are in
perfect agreement, strongly supporting the correctness of the proposed S
matrix. The mass gap is also computed in terms of the Lambda parameters of the
modified minimal substraction scheme and a lattice regularized version of the
model.
