T-dual R-R zero-norm states, D-branes and S-duality of type II string   theory
  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. 
Jen-Chi Lee