  We calculate the R-R zero-norm states of type II string spectrum. To fit
these states into the right symmetry charge parameters of the gauge
transformations of the R-R tensor forms, one is forced to T-dualize some type I
open string space-time coordinates and thus to introduce D-branes into the
theory. We also demonstrate that the constant T-dual R-R 0-form zero-norm
state, together with the NS-NS singlet zero-norm state are responsible for the
SL(2,Z) S-duality symmetry of the type II B string theory.
