  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 k_i. We expect that these relations will be useful
for a more precise determination of the electromagnetic low-energy constants.
