  We extend the usual chiral perturbation theory framework ($\chi$PT) to allow
the inclusion of a light dynamical isosinglet scalar. Using lattice QCD
results, and a few phenomenological inputs, we explore the parameter space of
the effective theory. We discuss the S-wave pion-pion scattering lengths,
extract the average value of the two light quark masses and evaluate the impact
of the dynamical singlet field in the low--energy constants $\bar{l}_1$,
$\bar{l}_3$ and $\bar{l}_4$ of $\chi$PT. We also show how to extract the mass
and width of the sigma resonance from chiral extrapolations of lattice QCD
data.
