  We study L-point couplings between twisted sector fields in heterotic
orbifold compactifications, using conformal field theory. Selection rules
provide an easy way to identify which couplings are non-vanishing. Those used
in the current literature are gauge invariance, R-charge conservation and the
space group selection rule, but they are not the whole story. We revive and
refine a fourth selection rule, due to symmetries in the underlying torus
lattice, and introduce a fifth one, due to the existence or not of classical
worldsheet instanton solutions to mediate the couplings. We consider briefly
the phenomenological consequences of the additional rules, in particular for
recent orbifold constructions whose field content correspond to that of the
MSSM. The structure of the exotic mass matrices is unaltered and many
dimension-5 proton-decay operators vanish.
