  We study the duality between four-dimensional N=2 compactifications of
heterotic and type IIA string theories. Via adiabatic fibration of the duality
in six dimensions, type IIA string theory compactified on a K3-fibred
Calabi-Yau threefold has a potential heterotic dual compactification. This
adiabatic picture fails whenever the K3 fibre degenerates into multiple
components over points in the base of the fibration. Guided by monodromy, we
identify such degenerate K3 fibres as solitons generalizing the NS5-brane in
heterotic string theory. The theory of degenerations of K3 surfaces can then be
used to find which solitons can be present on the heterotic side. Similar to
small instanton transitions, these solitons escort singular transitions between
different Calabi-Yau threefolds. Starting from well-known examples of
heterotic--type IIA duality, such transitions can take us to type IIA
compactifications with unknown heterotic duals.
