  Strong electron-phonon interaction in cuprates has gathered support over the
last decade in a number of experiments. While phonons remain almost
unrenormalised, electrons are transformed into itinerent bipolarons and
thermally excited polarons when the electron-phonon interaction is strong. We
calculate the Lorenz number of the system to show that the Wiedemann-Franz law
breaks down because of the interference of polaron and bipolaron contributions
in the heat flow. The model fits numerically the experimental Hall Lorenz
number, which provides a direct evidence for bipolarons in the cuprates.
