  We construct the six-dimensional Lagrangian for the massless twisted open
strings with one end-point ending on a stack of D5 and the other on a stack of
D9 branes, interacting with the gauge multiplets living respectively on the D5
and D9 branes. It is first obtained by uplifting to six dimensions the
four-dimensional Lagrangian of the N=2 hypermultiplet and manifestly exhibits
an SU(2) symmetry. We show by an explicit calculation that it is N=1
supersymmetric in six dimensions and then we check various terms of this
Lagrangian by computing string amplitudes on the disk. Finally, starting from
this Lagrangian and assuming the presence of non-zero magnetic fluxes along the
extra compact dimensions, we determine the spectrum of the Kaluza-Klein states
which agrees with the corresponding one obtained from string theory in the
field theory limit.
