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