Morrison, David R.Vafa, Cumrun2019-09-221996Morrison, David R., and Cumrun Vafa. 1996. “Compactifications of F-Theory on Calabi-Yau Threefolds (II).” Nuclear Physics B 476 (3): 437–69. https://doi.org/10.1016/0550-3213(96)00369-0.0169-68230550-32131873-1562http://nrs.harvard.edu/urn-3:HUL.InstRepos:41385064We continue our study of compactifications of F-theory on Calabi-Yau threefolds. We gain more insight into F-theory duals of heterotic strings and provide a recipe for building F-theory duals for arbitrary heterotic compactifications on elliptically fibered manifolds. As a byproduct we find that string/string duality in six dimensions gets mapped to fiber/base exchange in F-theory. We also construct a number of new N = 1, d = 6 examples of F-theory vacua and study transitions among them. We find that some of these transition points correspond upon further compactification to 4 dimensions to transitions through analogues of Argyres-Douglas points of N = 2 moduli. A key idea in these transitions is the notion of classifying (0, 4) fivebranes of heterotic strings.en-USJournal Article2019-09-2210.1016/0550-3213(96)00369-0