  We investigate pion photoproduction off the proton in a manifestly
gauge-invariant chiral unitary extension of chiral perturbation theory. In a
first step, we consider meson-baryon scattering taking into account all
next-to-leading order contact interactions. The resulting low-energy constants
are determined by a fit to s-wave pion-nucleon scattering and the low-energy
data for the reaction pi- p --> eta n. To assess the theoretical uncertainty,
we perform two different fit strategies. Having determined the low-energy
constants, we then analyse the data on the s-wave multipole amplitudes E0+ of
pion and eta photoproduction. These are parameter-free predictions, as the two
new low-energy constants are determined by the neutron and proton magnetic
moments.
