  It is shown, with the example of the experimentally known Adler function,
that there is no matching in the intermediate region between the two asymptotic
regimes described by perturbative QCD (for the very short-distances) and by
chiral perturbation theory (for the very long-distances). We then propose to
consider an approximation of large-Nc QCD which consists in restricting the
hadronic spectrum in the channels with J^P quantum numbers 0^-, 1^-, 0^+ and
1^+ to the lightest state and treating the rest of the narrow states as a
perturbative QCD continuum; the onset of this continuum being fixed by
consistency constraints from the operator product expansion. We show how to
construct the low-energy effective Lagrangian which describes this
approximation. The number of free parameters in the resulting effective
Lagrangian can be reduced, in the chiral limit where the light quark masses are
set to zero, to just one mass scale and one dimensionless constant to all
orders in chiral perturbation theory. A comparison of the corresponding
predictions, to O(p^4) in the chiral expansion, with the phenomenologically
known couplings is also made.
