  We show that the massless integrable sector of the AdS_3 \times S^3 \times
T^4 superstring theory, which admits a non-trivial relativistic limit, provides
a setting where it is possible to determine exact minimal solutions to the form
factor axioms, in integral form, based on analiticity considerations, along the
same lines of ordinary relativistic integrable models. We construct in full
detail the formulas for the two- and three-particle case, and show the
similarities as well as the differences with respect to the off-shell Bethe
ansatz procedure of Babujian et al. We show that our expressions pass a series
of non-trivial consistency checks which are substantially more involved than in
the traditional case. We speculate on the problems concerned in a possible
generalisation to an arbitrary number of particles, and on a possible
connection with the hexagon programme.
