  In this paper we derive from field theory a L\"uscher-formula, which gives
the leading exponentially small in volume corrections to the 1-particle
form-factors in non-diagonally scattering integrable quantum field theories.
Our final formula is expressed in terms of appropriate expressions of 1- and
3-particle form-factors, and can be considered as the generalization of
previous results obtained for diagonally scattering bosonic integrable quantum
field theories. Since our formulas are also valid for fermions and operators
with non-zero Lorentz-spin, we demonstrated our results in the Massive Thirring
Model, and checked our formula against 1-loop perturbation theory finding
perfect agreement.
