  We address non-perturbative effects and duality symmetries in $N=2$ heterotic
string theories in four dimensions. Specifically, we consider how each of the
four lines of enhanced gauge symmetries in the perturbative moduli space of
$N=2$ $T_2$ compactifications is split into 2 lines where monopoles and dyons
become massless. This amounts to considering non-perturbative effects
originating from enhanced gauge symmetries at the microscopic string level. We
show that the perturbative and non-perturbative monodromies consistently lead
to the results of Seiberg-Witten upon identication of a consistent truncation
procedure from local to rigid $N=2$ supersymmetry.
