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We study L-point couplings between twisted sector fields in heteroticorbifold compactifications, using conformal field theory. Selection rulesprovide an easy way to identify which couplings are non-vanishing. Those usedin the current literature are gauge invariance, R-charge conservation and thespace group selection rule, but they are not the whole story. We revive andrefine a fourth selection rule, due to symmetries in the underlying toruslattice, and introduce a fifth one, due to the existence or not of classicalworldsheet instanton solutions to mediate the couplings. We consider brieflythe phenomenological consequences of the additional rules, in particular forrecent orbifold constructions whose field content correspond to that of theMSSM. The structure of the exotic mass matrices is unaltered and manydimension-5 proton-decay operators vanish.

Revisiting Coupling Selection Rules in Heterotic Orbifold Models