A recently proposed set of sum rules, based on the pion-Kaon scattering
amplitudes and their crossing-symmetric conjugates are analysed in detail. A
key role is played by the $l=0$ $\pi\pi\to K\overline K$ amplitude which
requires an extrapolation to be performed. It is shown how this is tightly
constrained from analyticity, chiral counting and the available experimental
data, and its stability is tested. A re-evaluation of the $O(p^4)$ chiral
couplings $L_1$, $L_2$, $L_3$ is obtained, as well as a new evaluation of the
large $N_c$ suppressed coupling $L_4$.Comment: 28 pages, 7 figures [v1: small corrections and improvements