Exact massive S-matrices for two dimensional sigma models on symmetric spaces
SU(2N)/Sp(N) and Sp(2P)/Sp(P)*Sp(P) are conjectured. They are checked by
comparison of perturbative and non perturbative TBA calculations of free energy
in a strong external field. We find the mass spectrum of the models and
calculate their exact mass gap.Comment: 11 p., minor correction