  The gauge-fixed action of a `spacetime-filling' D3-brane with dilaton-axion
coupling is formulated in N=1 superspace. We investigate its symmetries by
paying special attention to a possible non-linearly realized extra (hidden)
supersymmetry, and emphasize the need of a linear superfield coupled to an
abelian Chern-Simons superfield to represent a dilaton-axion supermultiplet in
the off-shell manifestly supersymmetric approach.
