and the number of contact terms. For most processes of interest, the tree-level contributions at NNLO are small, since the relevant scale in most of these cases is $M_\pi$.

This has the advantage that the determination of the NLO LECs does not depend on how well we know the values of the $c_i^r$ but it makes comparison with models for the $c_i$ more uncertain.

All observables needed are known to NNLO. The NLO results are all present in [@Gasser:1983yg]. The NNLO results for $M_\pi^2$ and $F_\pi$ were done in [@Burgi:1996qi; @Bijnens:1997vq]. $\pi\pi$ scattering was done to NNLO in [@Bijnens:1995yn; @Bijnens:1997vq]. Finally, the scalar and vector form factors of the pion were obtained to two-loop order in [@Bijnens:1998fm] while the pion radiative decay can be found in [@Bijnens:1996wm].

The pion mass is experimentally very well known [@Beringer:1900zz]. The larger question here is which pion mass to use, charged or neutral. When comparing theoretical results with experimental quantities it is usually better to use the charged pion mass since most experiments are performed with these. When extracting quark masses, it is better to use the neutral pion mass since this is expected to have only a very small contribution from electromagnetism. In 