RELEVANT PERTURBATIONS OF THE SU(1, 1) U(1) COSET/

It is shown that the space of cohomology classes of the SU(1,1)/U(1) c oset at negative level k contains states of relevant conformal dimensi ons. These states correspond to the energy density operator of the ass ociated nonlinear sigma model. We exhibit that there exists a subclass of relevant operators forming a closed fusion algebra. We make use of these operators to perform renormalizable perturbations of the SU(1,1 )/U(1) coset. In the infra-red limit, the perturbed theory flows to an other conformal model. We identify one of the perturbative conformal p oints with the SU(2)/U(1) coset at positive level. From the point of v iew of the string target space geometry, the given renormalization gro up flow maps the noncompact geometry described by the SU(1,1)/U(1) cos et into the sphere described by the SU(2)/U(1) coset. This exhibits a new mechanism of topology change in the space of string compactificati ons.