Institute for Nuclear Research, Kyiv, UKRAINE
E-mail: malyuta@mail.kar.net

D-branes, orbifolds, and Ext groups

We investigate stability of D-branes on Calabi-Yau manifolds. It is shown that the octahedral axiom from the theory of derived categories is an essential ingredient in the study of  stability. Various examples in the context of Calabi-Yau threefolds are studied and we derive the walls of marginal stability in several cases.