The strong version of Maldacena's AdS/CFT conjecture implies that the large N expansion of free N = 4 super-YM theory describes an interacting string theory in the extreme limit of high spacetime curvature relative to the string length. String states may then be understood as composed of SYM string bits. We investigate part of the low-lying spectrum of the tensionless (zero-coupling) limit and find a large number of states that are not present in the infinite tension (strong-coupling) limit, notably several massless spin two particles. We observe that all conformal dimensions are N -independent in the free SYM theory, implying that masses in the corresponding string theory are unchanged by string interactions. Degenerate string states do however mix in the interacting string theory because of the complicated N -dependence of general CFT two-point functions. Finally we verify the CFT crossing symmetry, which corresponds to the dual properties of string scattering amplitudes. This means that the SYM operator correlation functions define AdS dual models analogous to the Minkowski dual models that gave rise to string theory.

We discuss the non-linear sigma model representing a NSR open string in a curved background with non-zero B µν -field. With this coupling the theory is not automatically supersymmetric, due to boundary contributions. When B = 0 supersymmetry is ensured by the conditions that follow as the boundary contribution to the field equations. We show that inclusion of a particular boundary term restores this state of affairs also in the presence of a B-field. The boundary conditions derived from the field equations in this case agree with those that have been proposed for constant B-field. A coupling to a boundary A µ -field will modify both the boundary conditions and affect the supersymmetry. It is shown that there is an A-coupling with non-standard fermionic part that respects both the supersymmetry and the shift symmetry (in the B and A fields), modulo the (modified) boundary conditions.

The technique of generating new solutions to 4D gravity-matter systems by dimensional reduction to a -model is extended to supersymmetric configurations of supergravity. The conditions required for the preservation of supersymmetry under isometry transformations in the -model target space are found. Some examples illustrating the technique are given.