  For heterotic string theory compactified on T^6, we derive the complete set
of T-duality invariants which characterize a pair of charge vectors (Q,P)
labelling the electric and magnetic charges of the dyon. Using this we can
identify the complete set of dyons to which the previously derived degeneracy
formula can be extended. By going near special points in the moduli space of
the theory we derive the spectrum of quarter BPS dyons in N=4 supersymmetric
gauge theory with simply laced gauge groups. The results are in agreement with
those derived from field theory analysis.
