# Why cannot a fundamental string couple to the R-R gauge field $C_{\mu\nu}$?

People usually say that D-branes can carry R-R charges, or can couple to R-R sector gauge fields. But why a fundamental string cannot couple to a 2-form R-R sector gauge field? What's the essential different between a D-string (D1-brane) and a fundamental string?

$$< closed| C_{(p+1)} |closed >$$