Low-energy expansion of meson form factors

We calculate the corrections to various low-energy theorems concerning the behaviour of the pseudoscalar meson form factors near t = 0. In particular we discuss (i) the Ademollo-Gatto theorem, (ii) Sirlin's relation between the Ksub(l3) form factor fsub(+)sup(Kπ) (t) and the electromagnetic form factors, (iii) the Callan-Treiman relation, and (iv) the Dashen-Weinstein relation, which connects the slope lambda0 of f0sup(Kπ)(t) with the ratio Fsub(K)/Fsub(π). Furthermore, we point out a remarkable isospin breaking effect which is clearly visible in the experimental rates of the decays K+ -> π0e+ν, K0 -> π-e+ν. (orig.)