  We propose a method to derive the low-energy efective action of QCD assuming
that the long-distance properties of strong interactions can be described by a
string theory. We bypass the usual problems related to the existence of the
tachyon and absence of the adequate Adler zero by using a sigma model approach
where excitations above the correct (chirally non-invariant) QCD vacuum are
included. Two-dimensional conformal invariance then implies the vanishing of
the O(p^4) effective lagrangian coefficients. We discuss ways to go beyond this
limit.
