  One-loop unitarized Chiral Perturbation Theory (UChPT) calculations, suggest
a different Nc behaviour for the sigma or f0(600) and rho(770) mesons: while
the rho meson becomes narrower with Nc, as expected for a qqbar meson, the
f0(600) contribution to the total cross section below 1 GeV becomes less and
less important. Here we review our recent work where we have shown, by means of
finite energy sum rules, that a different Nc behaviour for these resonances may
lead to a conflict with semi-local duality for large Nc, since local duality
requires a cancellation between the f0(600) and rho(770) amplitudes. However,
UChPT calculations also suggest a subdominant qqbar component for the f0(600)
with a mass above 1 GeV and this can restore semi-local duality, as we show.
