 Volume-coupling relation for M-theory on K3

A wrapped M5-brane fixes the heterotic string tension proportional to $V/{\ell }_p^6$. Reduction of the gravitational coupling gives $G_7\propto {\ell }_p^9/V$. Therefore $G_7/{\ell }_{s,H}^5\propto (V/{\ell }_p^4{)}^{3/2}$, yielding the displayed dimensionless coupling relation. This derivation separates the K3 volume modulus from its shape Narain moduli space.