We show that a ${\mathrm{Z}}_2$⊗${\mathrm{Z}}_3$ symmetry leads to the radiative generation of naturally small Dirac neutrino masses in a class of superstring theories. This model realizes in a simple and consistent way a recent suggestion by Masiero, Nanopoulos, and Sanda.

• Received 8 December 1986

DOI:https://doi.org/10.1103/PhysRevLett.58.969