Canonical Cartan connection and holomorphic invariants on Engel CR   manifolds
  We describe a complete system of invariants for 4-dimensional CR manifolds of CR dimension 1 and codimension 2 with Engel CR distribution by constructing an explicit canonical Cartan connection. We also investigate the relation between the Cartan connection and the normal form of the defining equation of an embedded Engel CR manifold. 
Valerii Beloshapka, Vladimir Ezhov, Gerd Schmalz