CHAINS OF N=2, D=4 HETEROTIC TYPE-II DUALS

We report on a search for N = 2 heterotic strings that are dual candid ates of type II compactifications on Calabi-Yau threefolds described a s K3 fibrations. We find many new heterotic duals by using standard or bifold techniques. The associated type II compactifications fall into chains in which the proposed duals are heterotic compactifications rel ated one another by a sequential Higgs mechanism. This breaking in the heterotic side typically involves the sequence SU(4) --> SU(3) --> SU (2) --> 0, while in the type II side the weights of the complex hypers urfaces and the structure of the K3 quotient singularities also follow specific patterns. Some qualitative features of the relationship betw een each model and its dual can be understood by fiber-wise applicatio n of string-string duality.