Connection with torsion, parallel spinors and geometry of Spin(7) manifolds

We show that on every Spin(7)-manifold there always exists a unique linear connection with totally skew-symmetric torsion preserving a nontrivial spinor and the Spin(7) structure. We express its torsion and the Riemannian scalar curvature in terms of the fundamental 4-form. We present an exlicit formula for the Riemannian covariant derivative of the fundamental 4form in terms of its exterior differential. We show the vanishing of the  genus and obtain a linear relation between Betti numbers of a compact Spin(7) manifolds which are locally but not globally conformally equivalent to a space with closed fundamental 4-form. A general solution to the Killing spinor equations is presented. Running title: Geometry of Spin(7)-manifolds Subj. Class.: Special Riemannian manifolds, Spin geometry, String theory MS classification: 53C25; 53C27; 53C55; 81T30