  Let Q be a Dynkin quiver, and let P(Q) be the corresponding preprojective
algebra. Let I be a set of pairwise different indecomposable irreducible
components of varieties of P(Q)-modules such that generically there are no
extensions between these components. We show that the number of elements in I
is at most the number of positive roots of Q. Furthermore, we give a module
theoretic interpretation of Leclerc's counterexample to a conjecture of
Berenstein and Zelevinsky.
