  Polchinski's recent construction of Dirichlet-branes of R-R charges, together
with Witten's mechanism for forming bound states of both NS-NS charges and R-R
charges, provides a rigorous method to treat these dy-branes. We construct the
massless sector of boundary states of D-branes, as well as of dy-strings of
charge $(p,1)$. As a consequence, the string tension formula predicted by
duality in the type IIB theory is obtained.
