The Sen limit

F -theory compactifications on elliptic Calabi-Yau manifolds may be related to IIb compactifications by taking a certain limit in complex structure moduli space, introduced by A. Sen. The limit has been characterized on the basis of SL(2, Z) monodromies of the elliptic fibration. Instead, we introduce a stable version of the Sen limit. In this picture the elliptic Calabi-Yau splits into two pieces, a P 1 -bundle and a conic bundle, and the intersection yields the IIb space-time. We get a precise match between F -theory and perturbative type IIb. The correspondence is holographic, in the sense that physical quantities seemingly spread in the bulk of the F -theory Calabi-Yau may be rewritten as expressions on the log boundary. Smoothing the F -theory Calabi-Yau corresponds to summing up the D(-1)-instanton corrections to the IIb theory.