TYPE-IIB INSTANTON AS A WAVE IN 12 DIMENSIONS

0-brane of type-IIA string theory can be interpreted as a dimensional reduction of a gravitational wave in 11 dimensions. We observe that a similar interpretation applies also to the D-instanton background of t ype-IIB theory: It can be viewed as a reduction (along one spatial and one timelike direction) of a wave in a 12-dimensional theory. The ins tanton charge is thus related to a linear momentum in 12 dimensions. T his suggests that the instanton should play as important a role in typ e-IIB theory as the 0-brane is supposed to play in type-IIA theory.