Proper Matter Collineations of Plane Symmetric Spacetimes
  We investigate matter collineations of plane symmetric spacetimes when the energy-momentum tensor is degenerate. There exists three interesting cases where the group of matter collineations is finite-dimensional. The matter collineations in these cases are either four, six or ten in which four are isometries and the rest are proper. 
M. Sharif and Tariq Ismaeel