  We discuss the physical superstring correlation functions in type I theory
(or equivalently type II with orientifold) that compute real topological string
amplitudes. We consider the correlator corresponding to holomorphic derivative
of the real topological amplitude $G_\chi$, at fixed worldsheet Euler
characteristic $\chi$. This corresponds in the low-energy effective action to
N=2 Weyl multiplet, appropriately reduced to the orientifold invariant part,
and raised to the power $g'=-\chi+1$. We show that the physical string
correlator gives precisely the holomorphic derivative of topological amplitude.
Finally, we apply this method to the standard closed oriented case as well, and
prove a similar statement for the topological amplitude $F_g$.
