BPS preons and tensionless super-p-brane in generalized superspace
  Tensionless super-p-branes in a generalized superspace with additional tensorial central charge coordinates may provide an extended object model for BPS preons, i.e. for the hypothetical constituents of M-theory preserving 31 of 32 supersymmetries [hep-th/0101113]. 
Igor A. Bandos