Superembedding Approach and S-Duality. A unified description of superstring and super-D1-brane

It is proved that a basic superembedding equation for the 2-dimensional worldsheet superspace ${\mathrm{\S }}^{(2\mathrm{\mid }8+8)}$ embedded into D=10 type IIB superspace $M^{(10\mathrm{\mid }16+16)}$ provides a universal, S-duality invariant description of a fundamental superstring and super-D1-brane. We work out generalized action principle, obtain superfield equations of motion for both these objects and find how the S-duality transformations relate the superfield equations of superstring and super-D1-brane. The superembedding of 6-dimensional worldsheet superspace ${\mathrm{\S }}^{(6\mathrm{\mid }16)}$ into the D=10 type IIB superspace will probably provide a similar universal description for the set of type IIB super-NS5-brane, super-D5-brane and a Kaluza-Klein monopole (super-KK5-brane).