  The PVLAS collaboration has announced recently the observation dichroism and
birefringence of vacuum permeated by a intense magnetic field. Both are typical
predictions of the existence of light particles that, as the QCD axion,
interact with the electromagnetic field. The strength of the interaction as
inferred from the PVLAS measurements is however strongly excluded by
astrophysical arguments and axion Helioscopes since these particles, as
neutrinos, would be copiously produced in the interior of the Sun. In this
Thesis several refined models are presented where the astrophysical bounds are
severely relaxed, allowing the PVLAS particle interpretation. All these models
involve the existence of further new particles, in particular paraphotons and
millicharged particles. Also a model-independent study is performed, showing
that the reconciliation of the PVLAS particle interpretation implies new low
energy physics, usually accessible to precision experiments. Finally, if the
PVLAS particle would be parity even, the strongest exclusion bounds would come,
not from astrophysics, but from experiments searching for 5th type forces.
