  Method for computing scattering amplitudes of open strings with Dirichlet
boundary on one end and Neumann boundary condition on the other is described.
Vertex operator for these states are constructed using twist fields which have
been studied previously in the context of Ashkin-Teller model and strings on
orbifolds. Using these vertex operators, we compute the three- and four-point
scattering amplitudes for (5,9) strings on 5-branes and 9-branes. In the low
energy limit, these amplitudes are found to be in exact agreement with the
field theory amplitudes for supersymmetric Yang-Mills coupled to
hypermultiplets in 6-dimensions. We also consider the 1-brane 5-brane system
and compute the amplitude for a pair of (1,5) strings to collide and to escape
the brane as a closed string. (1,5) strings are found to be remarkably
selective in their coupling to massless closed strings in NS-NS sector; they
only couple to the dilaton.
