On the Pais--Treiman method to measure $\pi\pi$ phase shifts in $K_{e4}$   decays
  We evaluate theoretical uncertainties to the method of Pais and Treiman to measure the $\pi\pi$ phase shifts in $K_{e4}$ decays. We find that they are very small, below $1\%$. 
G. Colangelo, M. Knecht and J. Stern