Space-time singularities and the axion in the Poincaré coset models ISO(2,1)/H

By promoting an invariant subgroup H of ISO(2, 1) to a gauge symmetry of a WZWN action, we obtain the description of a bosonic string moving either in a curved 4-dimensional space-time with an axion field and curvature singularities or in 3-dimensional Minkowski space-time.