  A supersymmetric inflationary stage dominated by an $F$-term has the problem
that the flatness of the potential is spoiled by supergravity corrections, that
is the slow-roll parameter $\eta$ gets contributions of order unity. We show
that in $F$-term inflationary models based on strings there is natural way of
obtaining small values of $\eta$. This happens in models of hybrid inflation
based on orbifold constructions, in which a modulus $T$ field is responsible
for the large value of the potential during inflation, and a second field
$\phi$ with appropriate modular weight is responsible for the roll-over. We
illustrate the mechanism with a model in which the inflaton potential is
provided by gaugino condensation, leading to succesful inflation.
