  On a projective complex manifold, the Abelian group of Divisors maps
surjectively onto that of holomorphic line bundles (the Picard group). On a
$G_2$-manifold we use coassociative submanifolds to define an analogue of the
first, and a gauge theoretical equation for a connection on a gerbe to define
an analogue of the last. Finally, we construct a map from the former to the
later. Finally, we construct some coassociative submanifolds in twisted
connected sum $G_2$-manifolds.
