  Let (M,I,J,K) be a compact hypercomplex manifold admitting an HKT-metric.
Assume that the canonical bundle of (M,I) is trivial as a holomorphic line
bundle. We show that the holonomy of Obata connection on M is contained in
SL(n,H). In Appendix we apply these arguments to compact nilmanifolds equipped
with abelian hypercomplex structures, showing that such manifolds have holonomy
in SL(n,H).
