  We study the heterotic string compactified on K3 x T^2 near the line T=U,
where the effective action becomes singular due to an SU(2) gauge symmetry
enhancement. By `integrating in' the light W^\pm vector multiplets we derive a
quantum corrected effective action which is manifestly SU(2) invariant and
non-singular. This effective action is found to be consistent with a residual
SL(2,Z) quantum symmetry on the line T=U. In an appropriate decompactification
limit, we recover the known SU(2) invariant action in five dimensions.
