The phase of charged Nariai solutions

In this note, we compute the phase of the one-loop Euclidean path integral around charged Nariai solutions in 4 dimensions, including both metric and gauge field fluctuations. These solutions have a $S^2\times S^2$ geometry, and a magnetic flux in one of the spheres. For charges smaller than a critical value, the phase matches the result for the uncharged Nariai solution, and for charges bigger than that value, the phase is $i^3$. Our analytical calculation in the full 4D geometry matches the result obtained recently within a 2D dilaton gravity reduction. Along the way, we also develop a method of dealing with residue zero modes in the de Donder gauge.