The degeneracies of $1/4$ BPS states with unit torsion in heterotic string theory compactified on a six-torus are given in terms of the Fourier coefficients of the reciprocal of the Igusa cusp Siegel modular form $\Phi_{ 10}$ of weight $10$. We use the symplectic symmetries of the latter to construct a fine-grained Rademacher type expansion which expresses these BPS degeneracies as a regularized sum over residues of the poles of $1/\Phi_{ 10}$. The construction uses two distinct ${ \rm SL}(2, \mathbb{ Z})$ subgroups of ${ \rm Sp}(2, \mathbb{ Z})$ which encode multiplier systems, Kloosterman sums and Eichler integrals appearing therein. Additionally, it shows how the polar data are explicitly built from the Fourier coefficients of $1/\eta^{ 24}$ by means of a continued fraction structure.
Categories: hep-th math-ph math.MP math.NT