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