A relativistic generalisation of rigid motions
  Radar-holonomic congruences of wordlines are proposed as a weaker substitute for the too restrictive class of Born-rigid motions. The definition is expressed as a set of differential equations. Integrability conditions and Cauchy data are studied. We finally obtain an example of a radar-holonomic congruence containing a given worldline with a given value of the rotation on this line. 
J Llosa, A Molina and D Soler
gr-qc math-ph math.MP