  We discuss carefully the vertices which describe the dipole open strings and
closed strings on a D-brane with magnetic flux on a torus. Translation
invariance along closed cycles forces surprisingly closed string vertices
written in open string formalism to acquire Chan-Paton like matrices. Moreover
the one loop amplitudes have a single trace for the part of gauge group with
the magnetic flux. These peculiarities are also required by consistency of the
action of T-duality in the open string sector. In this way we can show to all
orders in perturbation theory the equivalence of the T-dual open string
theories, gravitational interactions included. We provide also a new and direct
derivation of the bosonic boundary state in presence of constant magnetic and
Kalb-Ramond background based on Sciuto-Della Selva-Saito vertex formalism.
