Abstract: I will discuss how "mixed" amplitudes (i.e. amplitudes with both closed and open strings) can be used to derive the geometrical backreaction of different D-brane bound states. As an example, I will mainly focus on a particular class of 1/8-BPS configurations in type IIB and derive from string theory several non-trivial features of the corresponding supegravity solution. I will also discuss a simple dynamical process in the case of 1/2-BPS configurations.