The paper by C.M. Hull and P.K. Townsend explores the duality symmetries of type II string theory compactified on a six-torus, which reduces to $N = 8$ supergravity. They show that the $E_7$ duality symmetry is broken by quantum effects to a discrete subgroup, $E_7(\mathbb{Z})$, which includes both the T-duality group $O(6, 6; \mathbb{Z})$ and the S-duality group $SL(2; \mathbb{Z})$. The authors present evidence for the conjecture that $E_7(\mathbb{Z})$ is an exact 'U-duality' symmetry of type II string theory, which unifies the S and T dualities and mixes sigma-model and string coupling constants. They discuss the implications of this conjecture, including the identification of certain extreme black hole states with massive modes of the fundamental string and the coupling of gauge bosons from the Ramond-Ramond sector to solitons. The paper also examines similar issues in the context of toroidal string compactifications to other dimensions, compactifications of type II string on $K_3 \times T^2$, and compactifications of eleven-dimensional supermembrane theory.The paper by C.M. Hull and P.K. Townsend explores the duality symmetries of type II string theory compactified on a six-torus, which reduces to $N = 8$ supergravity. They show that the $E_7$ duality symmetry is broken by quantum effects to a discrete subgroup, $E_7(\mathbb{Z})$, which includes both the T-duality group $O(6, 6; \mathbb{Z})$ and the S-duality group $SL(2; \mathbb{Z})$. The authors present evidence for the conjecture that $E_7(\mathbb{Z})$ is an exact 'U-duality' symmetry of type II string theory, which unifies the S and T dualities and mixes sigma-model and string coupling constants. They discuss the implications of this conjecture, including the identification of certain extreme black hole states with massive modes of the fundamental string and the coupling of gauge bosons from the Ramond-Ramond sector to solitons. The paper also examines similar issues in the context of toroidal string compactifications to other dimensions, compactifications of type II string on $K_3 \times T^2$, and compactifications of eleven-dimensional supermembrane theory.