On Counting Flat Connections over $G_2$-Orbifolds

We study the moduli space of $G_2$-instantons on (projectively) flat bundles over torsion-free $G_2$-orbifolds. We prove that the moduli space is compact and smooth at the irreducible locus after adding small and generic holonomy perturbations. Consequently, we define an integer-valued invariant that is invariant under $C^0$-deformation of torsion-free $G_2$-structures. We compute this invariant for some orbifolds that arise in Joyce’s construction of compact $G_2$-manifolds