FUNDAMENTAL STRINGS AND D-STRINGS IN THE IIB MATRIX MODEL

The matrix model for IIB superstring proposed by Ishibashi, Kawai, Kit azawa and Tsuchiya is investigated. Consideration of planar and non-pl anar diagrams suggests that large-iii perturbative expansion is consis tent with the double scaling limit proposed by the above authors. We w rite down a Wilson loop that can be interpreted as a fundamental strin g vertex operator. The one-point tadpole in the presence of a D-string has the right form and this can be viewed as a matrix model derivatio n of the boundary conditions that define a D-string. We also argue tha t if worldsheet coordinates sigma and tau are introduced to the fundam ental string, then the conjugate variable d/d sigma and d/d tau can be interpreted as the D-string worldsheet coordinates. In this way the S L(2Z) duality group of the IIB superstring becomes identified with the symplectic group acting on (p,q).