  We show that a coupling between chameleon-like scalar fields and photons
induces linear and circular polarization in the light from astrophysical
sources. In this context chameleon-like scalar fields includes those of the
Olive-Pospelov (OP) model describing a varying fine structure constant. We
determine the form of this polarization numerically and give analytic
expressions in two useful limits. By comparing the predicted signal with
current observations we are able to improve the constraints on the
chameleon-photon coupling and the coupling in the OP model by over two orders
of magnitude. It is argued that, if observed, the distinctive form of the
chameleon induced circular polarization would represent a smoking gun for the
presence of a chameleon. We also report a tentative statistical detection of a
chameleon-like scalar field from observations of starlight polarization in our
galaxy.
