On the Lagrangian Realization of Non-Critical ${\cal W}$-Strings
  A large class of non-critical string theories with extended worldsheet gauge symmetry are described by two coupled, gauged Wess-Zumino-Witten Models. We give a detailed analysis of the gauge invariant action and in particular the gauge fixing procedure and the resulting BRST symmetries. The results are applied to the example of ${\cal W}_3$ strings. 
Alex Deckmyn, Ruud Siebelink, Walter Troost and Alexander Sevrin