Spin and Center of Mass in Axially Symmetric Einstein-Maxwell Spacetimes
  We give a definition and derive the equations of motion for the center of mass and angular momentum of an axially symmetric, isolated system that emits gravitational and electromagnetic radiation. A central feature of this formulation is the use of Newman-Unti cuts at null infinity that are generated by worldlines of the spacetime. We analyze some consequences of the results and comment on the generalization of this work to general asymptotically flat spacetimes. 
Carlos N. Kozameh, Gonzalo Quiroga
gr-qc