On the Frobenius stable part of Witt vector cohomology

For a proper (not necessarily smooth) variety over a finite field with q elements, Berthelot-Bloch-Esnault proved a trace formula which computes the number of rational points modulo q in terms of the Witt vector cohomology. We show the analogous formula for Witt vector cohomology of finite length. In addition, we prove a vanishing result for the compactly supported 'etale cohomology of a constant p-torsion sheaf on an affine Cohen-Macaulay variety in positive characteristic p.