Relations between SU(2)- and SU(3)-LECs in chiral perturbation theory

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Chiral perturbation theory in the two--flavour sector allows one to analyse Green functions in QCD in the limit where the strange quark mass is considered to be large in comparison to the external momenta and to the light quark masses mᵤ and md. In this framework, the low--energy constants of SU2ᵣ × SU2ₗ depend on the value of the heavy quark masses. For the coupling constants which occur at order p² and p⁴ in the chiral expansion, we worked out the dependence on the strange quark mass at two--loop accuracy, and provided analogous relations for some of the couplings cᵢ which are relevant at order p⁶. This talk comments on the methods used, and illustrates implications of the results obtained.