QCD isospin breaking in meson masses, decay constants and quark mass ratios

The procedure to calculate masses and matrix-elements in the presence of mixing of the basis states is explained in detail. We then apply this procedure to the two-loop calculation in Chiral Perturbation Theory of pseudoscalar masses and decay constants including quark mass isospin breaking. These results are used to update our analysis of K-l4 done previously and obtain a value of m(u)/m(d) in addition to values for the low-energy-constants L-i(r). (C) 2001 Published by Elsevier Science B.V.