  We study the space of geometric and open string moduli of type IIB
compactifications from the perspective of complex structure deformations of
F-theory. In order to find a correspondence, we work in the weak coupling limit
and for simplicity focus on compactifications to 6 dimensions. Starting from
the topology of D7-branes and O7-planes, we construct the 3-cycles of the
F-theory threefold. We achieve complete agreement between the degrees of
freedom of the Weierstrass model and the complex structure deformations of the
elliptic Calabi-Yau. All relevant quantities are expressed in terms of the
topology of the base space, allowing us to formulate our results for general
base spaces.
