Seven-branes and instantons in type IIB supergravity

F-theory provides a means to study the type IIB string theory in the non-perturbative regime where the complex coupling is of order unity. This should be contrasted with the perturbative regime where the string coupling is small. F-theory will be used in this thesis to argue for the existence of novel types of branes, called Q7-branes. The notion of a Q7-brane leads to the notion of Q-instantons that are related to Q7-branes by electro-magnetic duality. Understanding the world-volume theory of the Q7-branes and the role of the Q-instantons provides a means to study the IIB theory in, so far, poorly investigated corners of its moduli space where the string coupling is of order unity. By moduli space is meant the set of inequivalent values of the complex type IIB coupling constant. From the set of one-half BPS objects that are present in type IIB supergravity almost all the branes have been given a microscopic interpretation in type IIB string theory. These are the branes that are referred as the (p',q') p-branes with p' and q' relatively prime integers. These are p-dimensional branes on which a (p',q') string is ending. The above-mentioned Q7-branes and Q-instantons are not of this type. The properties of the Q7-branes such as their gauge group, monodromy and mass have a natural interpretation in terms of coincident F-theory 7-branes, i.e. coinciding (p',q') 7-branes with different values for p' and q'. If the relative positions of the F-theory 7-branes that make up a Q7-brane are kept fixed so that they remain coincident and only the fluctuations associated to the center of mass motion are considered then the Q7-brane behaves effectively as a single brane that couples to an 8-form. By electro-magnetic duality it can be argued that there should exist a Q-instanton, i.e. a Q(-1)-brane that couples magnetically to the same 8-form to which a Q7-brane couples electrically. The path integral approach to the Q-instantons shows the existence of new vacua and a new superselection parameter. Further, it will be argued that the Q-instantons contribute to the R^4 terms of the string effective action near points in the moduli space where the string coupling is of order unity.