Non orientable three-submanifolds of $\mathrm{G}_2-$manifolds
  By analogy with associative and co-associative cases we introduce a class of three-dimensional non-orientable submanifolds, of almost $\mathrm{G}_2-$manifolds, modelled on planes lying in a special $\mathrm{G}_2-$orbit. An application of the Cartan-K\"ahler theory shows that some three-manifold can be presented in this way. We also classify all the homogeneous ones in $\mathbb{RP}^7$. 
Leonardo Bagaglini
math.DG