Custodial nonabelian gauge symmetries in realistic superstring derived models

A well known problem in supersymmetric models is the presence of, lepton and baryon number violating, dimension four operators. The traditional R parity solution may not be suitable if one tries to incorporate supersymmetry into a Planck scale theory. I propose a different solution in the context of realistic string models. I show that realistic string models can give rise to custodial nonabelian gauge symmetries under which only the leptons or quarks transform. I explain how such symmetries arise in a class of free fermionic models that are based on $Z_2\times Z_2$ orbifold with standard embedding. The custodial symmetries forbid proton decay from dimension four operators while allowing R parity violation.