Invariant Algebraic Slicing of the Spacetime
  Using the momentum constraint, the standard evolution system is written in a fully first order form. The class of first order invariant algebraic slicing conditions is considered. The full set of characteristic fields is explicitly given. Characteristic speeds associated to the gauge dependent eigenfields (gauge speeds) are related to light speed. 
Carles Bona, Joan Stela, Joan Masso, Edward Seidel