  We show that a generalization of $su(2)$ Chiral Perturbation Theory,
including a perturbative singlet scalar field, converges faster towards the
physical value of sensible low--energy observables. The physical mass and width
of the scalar particle are obtained through a simultaneous analysis of the pion
radius and the gamma gamma -> pi^0 pi^0 cross-section. Both values are
statistically consistent with the ones obtained by using Roy equations in pi-pi
scattering. In addition we find indications that the photon-photon-singlet
coupling is quite small.
