Some remarks on strong ${\mathrm{G}}_2$-structures with torsion

A ${\mathrm{G}}_2$-structure on a $7$-manifold $M$ is called a ${\mathrm{G}}_{2}T$-structure if $M$ admits a ${\mathrm{G}}_2$-connection ${\nabla }^T$ with totally skew-symmetric torsion $T_{\phi }$. If furthermore, $T_{\phi }$ is closed then it is called a strong ${\mathrm{G}}_{2}T$-structure. In this paper we investigate the geometry of (strong) ${\mathrm{G}}_{2}T$-manifolds in relation to its curvature, $S^1$ action and almost Hermitian structures. In particular, we study the Ricci flatness condition of ${\nabla }^T$ and give an equivalent characterisation in terms of geometric properties of the ${\mathrm{G}}_2$ Lee form. Analogous results are also obtained for almost Hermitian $6$-manifolds with skew-symmetric Nijenhuis tensor. Moreover, by considering the $S^1$ reduction by the dual of the ${\mathrm{G}}_2$ Lee form, we show that Ricci-flat strong ${\mathrm{G}}_{2}T$-structures correspond to solutions of the $\mathrm{SU}(3)$ heterotic system on certain almost Hermitian half-flat $6$-manifolds. Many explicit examples are described and in particular, we construct the first examples of strong ${\mathrm{G}}_{2}T$-structures with ${\nabla }^T$ not Ricci flat. Lastly, we classify ${\mathrm{G}}_2$-flows inducing gauge fixed solutions to the generalised Ricci flow akin to the pluriclosed flow in complex geometry. The approach is this paper is based on the representation theoretic methods due to Bryant.