Previous calculations of instanton effects in electroweak theory have
concentrated on the case of zero Weinberg angle θw​, where the U(1)
hypercharge field decouples. In this paper we extend the instanton calculation
to non-zero θw​, by constructing a perturbation expansion. This allows
for the first time the study of photon production at B and L number violating
verticies. We find that the orientation of the instanton solution in isospin
space has to be carefully considered to avoid unphysical results.Comment: uuencoded postscript, 10 pages