NS open strings with B fields and their interactions with NS closed strings

We consider interactions of an NS open string with the first important states of an NS closed string, i.e., a closed-string tachyon and a graviton, where both ends of the NS open string are attached to a D-brane, and a constant background B field lies along directions parallel to the D-brane world volume. Contrary to our expectations, we find that there are no constraints on these vertex operators resulting from the B field. However, we point out that these vertex operators have singularities at both ends of the NS open string when the external momenta take certain values. These kinds of singularities essentially result from the Dirichlet boundary conditions along directions transverse to the D-brane world volume.