NAMBU MASS HIERARCHIES IN LOW-ENERGY STRING MODELS

We show that a recent proposal by Nambu to generate a hierarchy among Yukawa couplings in the standard model may be easily implemented in su perstring models. In such models, two of the main ingredients of the N ambu proposal find a natural explanation: minimising with respect to t he Yukawas amounts to a minimisation with respect to the underlying mo duli fields and a constraint on the Yukawas of the type of the Veltman condition may be attributed to the relaxation process to a phenomenol ogically viable string vacuum.