  We show that the category of orbits of the bounded derived category of a
hereditary category under a well-behaved autoequivalence is canonically
triangulated. This answers a question by A. Buan, R. Marsh and I. Reiten which
appeared in their study with M. Reineke and G. Todorov of the link between
tilting theory and cluster algebras (closely related to work by
Caldero-Chapoton-Schiffler) and a question by H. Asashiba about orbit
categories. We observe that the resulting triangulated orbit categories provide
many easy examples of triangulated categories with the Calabi-Yau property.
These include the category of projective modules over a preprojective algebra
of generalized Dynkin type in the sense of Happel-Preiser-Ringel, whose
triangulated structure goes back to Auslander-Reiten's work on the
representation-theoretic approach to rational singularities.
