  We study two phases of a heterotic standard model, obtained from a Calabi-Yau
compactification of the E8xE8 heterotic string, in the context of the
associated four-dimensional effective theories. In the first phase we have a
standard model gauge group, an MSSM spectrum, four additional U(1) symmetries
and singlet fields. In the second phase, obtained from the first by continuing
along the singlet directions, three of the additional U(1) symmetries are
spontaneously broken and the remaining one is a B-L symmetry. In this second
phase, dimension five operators inducing proton decay are consistent with all
symmetries and as such, they are expected to be present. We show that, contrary
to this expectation, these operators are forbidden due to the additional U(1)
symmetries present in the first phase of the model. We emphasize that such
'unexpected' absences of operators, due to symmetry enhancement at specific
loci in the moduli space, can be phenomenologically relevant and, in the
present case, protect the model from fast proton decay.
