The phenomenon of coreference, covering entities, their mentions and their properties, is intricately linked to the phenomenon of coherence, covering the structure of rhetorical relations in a discourse. A text corpus that has both phenomena annotated can be used to test hypotheses about their interrelation or to detect other phenomena. We present the process by which C-3, a new corpus, was obtained by annotating the Discourse GraphBank coherence corpus with entity and mention information. The annotation followed a set of ACE guidelines adapted to favor coreference and to include entities of unknown types in the annotation. Together with the corpus we offer a new annotation tool specifically designed to annotate entity and mention information within a simple and functional graphical interface that combines the best of all worlds from available annotation tools. The potential usefulness of C-3 is discussed, as well as an application in which the corpus proved to be a valuable resource.