  We show that the exact N=1 superpotential of a class of 4d string
compactifications is computed by the closed topological string compactified to
two dimensions. A relation to the open topological string is used to define a
special geometry for N=1 mirror symmetry. Flat coordinates, an N=1 mirror map
for chiral multiplets and the exact instanton corrected superpotential are
obtained from the periods of a system of differential equations. The result
points to a new class of open/closed string dualities which map individual
string world-sheets with boundary to ones without. It predicts an
mathematically unexpected coincidence of the closed string Gromov-Witten
invariants of one Calabi-Yau geometry with the open string invariants of the
dual Calabi-Yau.
