A Note on Closed G_2-Structures and 3-Manifolds
  This article shows that given any orientable 3-manifold X, the 7-manifold T^*X x R admits a closed G_2-structure varphi=Re(Omega)+omega\wedge dt where Omega is a certain complex-valued 3-form on T^*X; next, given any 2-dimensional submanifold S of X, the conormal bundle N^*S of S is a 3-dimensional submanifold of T^*X x R such that varphi restricted to N^*S is equivalent to 0. A corollary of the proof of this result is that N^*S x R is a 4-dimensional submanifold of T^*X x R such that varphi restricted to N^*S x R is equivalent to 0. 
Hyunjoo Cho, Sema Salur and Albert J. Todd
math.DG hep-th math.SG