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The microscopic spectrum of half-BPS excitations in toroidally compactifiedheterotic string theory has been computed exactly through the use of resultsfrom analytic number theory. Recently, similar quantities have been understoodmacroscopically by evaluating the gravitational path integral on the M-theorylift of the AdS2 near-horizon geometry of the corresponding black hole. In thispaper, we generalize these results to a subset of the CHL models, which includethe standard compactification of IIA on $K3\times T^2$ as a special case. Webegin by developing a Rademacher-like expansion for the Fourier coefficients ofthe partition functions for these theories, which are modular forms forcongruence subgroups. We then interpret these results in a macroscopic settingby evaluating the path integral for the reduced-rank $\mathcal{N} = 4$supergravities described by these CFTs.

Exact Half-BPS Black Hole Entropies in CHL Models from Rademacher Series