In these lectures a general introduction to T-duality is given. In the abelian case the approaches of Buscher, and Rocek and Verlinde are reviewed. Buscher's prescription for the dilaton transformation is recovered from a careful definition of the gauge integration measure. It is also shown how duality can be understood as a quite simple canonical transformation. Some aspects of non-abelian duality are also discussed, in particular what is known on relation to canonical transformations. Some implications of the existence of duality on the cosmological constant and the definition of distance in String Theory are also suggested.

We explore some of the global aspects of duality transformations in String Theory and Field Theory. We analyze in some detail the equivalence of dual models corresponding to different topologies at the level of the partition function and in terms of the operator correspondence for abelian duality. We analyze the behavior of the cosmological constant under these transformations. We also explore several examples of non-abelian duality where the classical background interpretation can be maintained for the original and the dual theory. In particular, we construct a non-abelian dual of SL(2, R) which turns out to be a three-dimensional black hole.

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We investigate the effective worldvolume theories of branes in a background given by (the bosonic sector of) 10-dimensional massive IIA supergravity ("massive branes") and their M-theoretic origin. In the case of the solitonic 5-brane of type IIA superstring theory the construction of the Wess-Zumino term in the worldvolume action requires a dualization of the massive Neveu-Schwarz/Neveu-Schwarz target space 2-form field. We find that, in general, the effective worldvolume theory of massive branes contains new worldvolume fields that are absent in the massless case, i.e. when the mass parameter m of massive IIA supergravity is set to zero. We show how these new worldvolume fields can be introduced in a systematic way.In particular, we find new couplings between the massive solitonic 5-brane and the target space background, involving an additional worldvolume 1-form and 6-form. These new couplings have implications for the anomalous creation of branes. In particular, when a massive solitonic 5-brane passes through a D8-brane a stretched D6-brane is created. Similarly, in M-theory we find that when an M5-brane passes through an M9-brane a stretched Kaluza-Klein monopole is created.We show that pairs of massive branes of type IIA string theory can be viewed as the direct and double dimensional reduction of a single "massive M-brane" whose worldvolume theory is described by a gauged sigma model. For D-branes, the worldvolume gauge vector field becomes the Born-Infeld field of the 10-dimensional brane. The construction of the gauged sigma model requires that the 11-dimensional background has a Killing isometry. This background can be viewed as an 11-dimensional rewriting of the 10-dimensional massive IIA supergravity theory. We present the explicit form and discuss the interpretation of (the bosonic sector of) this so-called "massive 11-dimensional supergravity theory".

We study AdS 3 × S 2 solutions in massive IIA that preserve small N = (4, 0) supersymmetry in terms of an SU(2)-structure on the remaining internal space. We find two new classes of solutions that are warped products of the form AdS 3 ×S 2 ×M 4 ×R. For the first, M 4 =CY 2 and we find a generalisation a D4-D8 system involving possible additional branes. For the second, M 4 need only be Kahler, and we find a generalisation of the T-dual of solutions based on D3branes wrapping curves in the base of an elliptically fibered Calabi-Yau 3-fold. Within these classes we find many new locally compact solutions that are foliations of AdS 3 × S 2 × CY 2 over an interval, bounded by various D brane and O plane behaviours. We comment on how these local solutions may be used as the building blocks of infinite classes of global solutions glued together with defect branes. Utilising T-duality we find two new classes of AdS 3 × S 3 × M 4 solutions in IIB. The first backreacts D5s and KK monopoles on the D1-D5 near horizon. The second is a generalisation of the solutions based on D3-branes wrapping curves in the base of an elliptically fibered CY 3 that includes non trivial 3-form flux.