On the Physical Meaning of the Robinson-Trautman-Maxwell Fields
  We study the Robinson-Trautman-Maxwell Fields in two closely related coordinate systems, the original Robinson-Trautman (RT) coordinates (in a more general context, often referred to as NU coordinates) and Bondi coordinates. In particular, we identify one of the RT variables as a velocity and then from the Bondi energy-momentum 4-vector, we find kinematic expressions for the mass and momentum in terms of this velocity. From these kinematic expressions and the energy-momentum loss equation we obtain surprising equations of motion for `the center of mass' of the source where the motion takes place in the four-dimensional Poincare translation sub-group of the BMS group. 
Carlos Kozameh, E.T. Newman, and Gilberto Silva-Ortigoza