Compactifications of F-theory on Calabi-Yau threefolds. (I) Original Research Article

We study compactifications of F-theory on certain Calabi-Yau threefolds. We find that N = 2 dualities of type II/heterotic strings in 4 dimensions get promoted to N = 1 dualities between heterotic string and F-theory in 6 dimensions. The six-dimensional heterotic/heterotic duality becomes a classical geometric symmetry of the Calabi-Yau in the F-theory setup. Moreover the F-theory compactifacation sheds light on the nature of the strong coupling transition and what lies beyond the transition at finite values of heterotic string coupling constant.