A STRING MODEL WITH SU(4)XO(4)X[SP(4)](HIDDEN) GAUGE-SYMMETRY

In the four dimensional free fermionic formulation of the heterotic st ring, a semi-realistic SU(4) x SU(2)(L) x SU(2)(R) model is proposed w ith three fermion generations in (4,2,1) + ((4) over bar,1,2) represen tations. The gauge symmetry of the model breaks to the standard gauge group using a Higgs pair in the (4,1,2) + ((4) over bar,1,2) represent ations. The massless spectrum includes exotic fractionally charged sta tes with non-trivial transformation properties under part (Sp(4)) of t he non-abelian 'hidden' symmetry. Finally there is a mirror pair in (4 ,2,1),((4) over bar,2,1) allowing the possibility for an identical run ning of g(4,L,R) couplings between the string and SU(4) breaking scale s. This is of crucial importance for a successful prediction of the we ak mixing angle. Potential shortcomings and problems of the constructi on are analysed and possible solutions are discussed.