The twistor space of a quaternionic contact manifold

We show that the CR structure on the twistor space of a quaternionic contact structure described by Biquard is normal if and only if the Ricci curvature of the Biquard connection commutes with the endomorphisms in the quaternionic structure of the contact distribution.Comment: 15pages, LaTeX2e, typos corrected, final version to appear in Quart. J. Math. (Oxford