(\[eq:l5l6\]) can be combined to [@GonzalezAlonso:2008rf] $$\label{eq:l6b}
\bar l_6 = 15.24\pm0.39\,,$$ nicely compatible within errors with (\[eq:l6\]), thus providing proof that [ChPT]{} works well in this sector. Another check that [ChPT]{} works was done by looking at the relations for $\pi\pi$ scattering found in [@Bijnens:2009zd] independent of the values of the $c_i^r$. Those were fairly well satisfied.

The determination of the constants at order $p^6$ is in worse shape. Four combinations of the $c_i^r$ are reasonably well known. These come from $c_V^\pi$, $c_S^\pi$ [@Bijnens:1998fm] and $\pi\pi$ scattering [@Colangelo:2001df]: $$\begin{aligned}
\label{eq:ci} 
r_{V2}^r &=& -4 c_{51}^r+4 c_{53}^r = (1.6\pm0.5)\cdot 10^{-4}\,,
\nonumber\\
{r_{S3}^r} &=& {-8 c_6^r \approx  1.5  \cdot 10^{-4}}\,,
\nonumber\\
r_5^r &=& -8 c_1^r+10 c_2^r+14 c_3^r = (1.5\pm0.4)\cdot 10^{-4}\,,
\nonumber\\
r_6^r &=& 6 c_2^r+2c_3^r = (0.40\pm0.04)\cdot 10^{-4}\,.\end{aligned}$$ $r_{S3}^r$ is not so well known since $c_S^\pi$ is dominated by the other contributions.

Including lattice results {#sec:nf2lattice}
-------------------------

In the last years the quark masses obtainable on the lattice have been coming closer to and even reaching the physical point. The situation relevant for the quantities considered in this review is presented in the FLAG reports [@Colangelo:2010et; @Aoki:2013ldr]. In particular, Sec. 5.1 of [@Aoki:2013ldr] reviews the status as of 2013.

The value of $\bar 