  We study three flavour chiral perturbation theory 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 this case, the theory reduces to the one of SU(2)_L x
SU(2)_R. Through this reduction, one can work out the strange quark mass
dependence of the LECs in the two-flavour case. We present the pertinent
relations at two-loop order for F,B and l_i.
