Extrinsic hyperspheres in manifolds with special holonomy
  We describe extrinsic hyperspheres and totally geodesic hypersurfaces in manifolds with special holonomy. In particular we prove the nonexistence of extrinsic hyperspheres in quaternion-Kaehler manifolds. We develop a new approach to extrinsic hyperspheres based on the classification of special Killing forms. 
Tillmann Jentsch, Andrei Moroianu, Uwe Semmelmann
math.DG