  We consider the physical implications of very light axiodilatons motivated by
a novel mechanism to substantially reduce the vacuum energy proposed in
arXiv:2110.10352. We address the two main problems concerning the light
axiodilaton that appears in the low-energy limit, namely that the axion has a
very low decay constant $f_a \sim $ eV (as read from its kinetic term) and that
the dilaton is subject to bounds that are relevant to tests of GR once
$\rho_{\rm vac} \leq 10^{-80} M_p^4$. We show that eV scale axion decay
constants need not be a problem by showing how supersymmetric extra dimensions
provide a sample unitarization for axion physics above eV scales for which
non-anomalous matter/axiodilaton couplings can really have gravitational
strength, showing how naive EFT reasoning can mistakenly overestimates axion
interactions at eV. When axions really do couple strongly at eV scales we
identify the dimensionless interaction in the UV completion that is also O(1),
and how axion energy-loss bounds map onto known extra-dimensional constraints.
We find a broad new class of exact exterior solutions to the vacuum axiodilaton
equations and knowledge of axiodilaton-matter couplings also allows us to
numerically search for interior solutions that match to known exterior
solutions that can evade solar-system tests. We find no examples that do so,
but also identify potential new candidate mechanisms for reducing the effective
dilaton-matter coupling to gravitating objects without also undermining the
underlying suppression of $\rho_{\rm vac}$.
