Exact Bethe ansatz solution of O(2N) symmetric theories

We construct exact eigenvectors and eigenvalues of O(2N) symmetric two-dimensional theories (vertex models and quantum field theories) by means of a generalization of the nested Bethe ansatz method. The spectrum of the physical states as well as the structure of the eigenvectors are explicitly derived. Their group theoretical properties are investigated, the free energy of the vertex model is given and its properties analysed. (orig.)