There are, actually. Dilaton (I don't mean the massless field in the NS-NS sector that determines the coupling constant, nor the $g_{55}$ component of the Kaluza-Klein Spacetime metric tensor) already covered the reason through T-duality, so I will discuss the requirement of $p$-branes imposed by Ramond-Ramond potentials.

The worldsheet of a string can couple to a Neveu-Schwarz B-field: $$q\int_{}^{} {{{h^{ab}}}\frac{{\partial {X^\mu }}}{{\partial {\xi ^a}}}\frac{{\partial {X^\nu }}}{{\partial {\xi ^b}}}B_{\mu \nu }\sqrt { - \det {h_{ab}}} {{\text{d}}^2}\xi } $$

Now, the $q$ is the EM-charge.

The worldsheet of a string can couple to graviton field (spacetime metric): $$m\int_{}^{} {{{h^{ab}}}\frac{{\partial {X^\mu }}}{{\partial {\xi ^a}}}\frac{{\partial {X^\nu }}}{{\partial {\xi ^b}}}g_{\mu \nu }\sqrt { - \det {h_{ab}}} {{\text{d}}^2}\xi } $$

You can change the "$m$" to any way you like, in terms of the tension/Regge Slope parameter/string length etc.

For a dilaton (now, I DO mean the massless field in the NS-NS sector which determines the coupling) field, $${q }\ell _P^2\int_{}^{} {\Phi R\sqrt { - \det {h_{\alpha \beta }}} {\text{ }}{{\text{d}}^2}\xi } $$ Forget the conformal invariance for the time being.

But what about Ramond-Ramond POTENTIALS? All is fine with the Ramond-Ramond Fields, but the Ramond-Ramond Potentials $C_k$are associated with the Ramond-Ramond field $A_{k+1}$ and it is intuitive (and quite clear) that they can't couple similarly to the worldsheet. But it can for a worldhhypervolume, as long as the world-hypervolume is not 2-dimensional. It would then be given by: $${q_{{\text{RR}}}}\int_{}^{} {C_{{\mu _1}...{\mu _p}}^{p + 1}\frac{{\partial {x^{{\mu _1}}}}}{{\partial {\xi ^{{a_1}}}}}...\frac{{\partial {x^{{\mu _p}}}}}{{\partial {\xi ^{{a_p}}}}}{h^{{a_0}...{a_p}}}\sqrt { - \det {h^{{a_0}...{a_p}}}} {{\text{d}}^{p + 1}}\xi } $$

Just note its similarity to the other couplings! Now, maybe this isn't so much of a necessity .as T-duality's switching of Newmann and Dirchilets, but it is still very important!

**Edit:** It works with 2-branes (membranes) too, but there's no point stopping there, and t-duality exchanging boundary condition becomes an issue. While in 10-dimensional string theories, all these branes are consistent, in 11-dimensional M-theory, only 2-branes and 5-branes are.