Proton stability in superstring derived models

I discuss the problem of proton decay, from dimension-four, -five and -six operators, in superstring derived standard-like models. I classify the sectors that produce color triplet superfields which may generate proton decay from dimension-five and -six operators. I show that for two of these sectors there exists a unique superstring doublet-triplet splitting mechanism that projects out the color triplets by means of the GSO projection, while leaving the electroweak doublets in the physical spectrum. I investigate possible proton decay due to additional color triplets in the massless spectrum and due to non-renormalizable terms I show that there exist models in which the dimension-four, -five and -six operators, that lead to proton decay, are forbidden by the symmetries of the string models, to all orders of nonrenormalizable terms. ((orig.))