  I discuss different aspects of the phenomenology of hypothetical sub eV mass
particles arising in the context of extensions of the standard model. I focus
on a simple extension based on an additional U(1) gauge symmetry and its
corresponding gauge boson, called ``hidden photon''. Kinetic mixing with the
standard photon leads to photon-hidden photon oscillations that are searched
for in laboratory experiments like ALPS at DESY. Hidden photons produced in the
interior of the Sun could be also detected in axion helioscopes like CAST at
CERN and could play an interesting role in late cosmology, where the presence
of additional feebly interacting relativistic particles seems to be favored.
All these effects disappear as the hidden photon mass decreases, allowing
phenomenologically large kinetic mixings. However, in this case such a hidden
photon will even play a role in gauge coupling unification.
