  This work focuses on the calculation of the large-volume behaviour of form
factors of local operators in the XXZ spin-$1/2$ chain taken between the ground
state and an excited state containing bound states. The analysis is rigorous
and builds on various fine properties of the string solutions to the Bethe
equations and certain technical hypotheses. These technical hypotheses are
satisfied for a generic excited state. The results obtained in this work pave
the way for extracting, starting from the first principles, the large-distance
and long-time asymptotic behaviour of the XXZ chain's two-point functions just
as the so-called edge singularities of their Fourier transforms.
