ar X iv : 0 71 0 . 54 32 v 1 [ he p - ph ] 2 9 O ct 2 00 7 IFIC / 07 – 53 Electromagnetic low – energy constants in χ pt

We investigate three–flavour chiral perturbation theory including virtual photons in a limit where the strange quark mass is much larger than the external momenta and the up and down quark masses, and where the external fields are those of two–flavour chiral perturbation theory. In particular we work out the strange quark mass dependence of the electromagnetic two–flavour low–energy constants C and ki. We expect that these relations will be useful for a more precise determination of the electromagnetic low–energy constants.