  If the recent PVLAS results on polarization changes of a linearly polarized
laser beam passing through a magnetic field are interpreted by an axion-like
particle, it is almost certain that it is not a standard QCD axion. Considering
this, we study the general effective interactions of photons with spin-zero
particles without restricting the latter to be a pseudo-scalar or a scalar,
i.e., a parity eigenstate. At the lowest order in effective field theory, there
are two dimension-5 interactions, each of which has previously been treated
separately for a pseudo-scalar or a scalar particle. By following the evolution
in an external magnetic field of the system of spin-zero particles and photons,
we compute the changes in light polarization and the transition probability for
two experimental set-ups: one-way propagation and round-trip propagation. While
the first may be relevant for astrophysical sources of spin-zero particles, the
second applies to laboratory optical experiments like PVLAS. In the one-way
propagation, interesting phenomena can occur for special configurations of
polarization where, for instance, transition occurs but light polarization does
not change. For the round-trip propagation, however, the standard results of
polarization changes for a pseudoscalar or a scalar are only modified by a
factor that depends on the relative strength of the two interactions.
