SUSY GAUGE DYNAMICS AND SINGULARITIES OF 4D N=1 STRING VACUA

Many N = 1 heterotic string compactifications exhibit physically myste rious singularities at codimension one in the moduli space of vacua. A t these singularities, Yukawa couplings of charged fields develop pole s as a function of the moduli, We explain these conformal field theory singularities, in a class of examples, as arising from non-perturbati ve gauge dynamics of non-perturbative gauge bosons (whose gauge coupli ng is the sigma model coupling) in the string theory.