  Light scalar fields can naturally couple disformally to Standard Model fields
without giving rise to the unacceptably large fifth forces usually associated
with light scalars. We show that these scalar fields can still be studied and
constrained through their interaction with photons, and focus particularly on
changes to the Cosmic Microwave Background spectral distortions and violations
of the distance duality relation. We then specialise our constraints to scalars
which could play the role of axionic quintessence.
