Strong phases in the decays B to ππ

The observed strong phase difference of 30\ifmmode^\circ\else\textdegree\fi{} between $I=\frac{3}{2}$ and $I=\frac{1}{2}$ final states for the decay $\stackrel{\ensuremath{\rightarrow}}{B}D\ensuremath{\pi}$ is analyzed in terms of rescattering like ${D}^{*}\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\rho}}D\ensuremath{\pi},$ etc. It is concluded that for the decay ${B}^{0}\ensuremath{\rightarrow}{D}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ the strong phase is only about 10\ifmmode^\circ\else\textdegree\fi{}. Implications for the determination of $\mathrm{sin}(2\ensuremath{\beta}+\ensuremath{\gamma})$ are discussed.