Real $K3$ surfaces without real points, equivariant determinant of the Laplacian, and the Borcherds Phi-function

We consider an equivariant analogue of a conjecture of Borcherds. Let $Y$ be a real $K3$ surface without real points. Let $g$ be a Ricci-flat Kaehler metric on $Y$ invariant under the complex conjugation. We shall prove that the equivariant determinant of the Laplacian of $(Y,g)$ with respect to the complex conjugation is expressed as the norm of the Borcherds Phi-function at the "period point". Here the period is not the one in algebraic geometry.