Helioscope Bounds on Hidden Sector Photons
  The flux of hypothetical "hidden photons" from the Sun is computed under the assumption that they interact with normal matter only through kinetic mixing with the ordinary standard model photon. Requiring that the exotic luminosity is smaller than the standard photon luminosity provides limits for the mixing parameter down to 10^-14, depending on the hidden photon mass. Furthermore, it is pointed out that helioscopes looking for solar axions are also very sensitive to hidden photons. The recent results of the CAST collaboration are used to further constrain the mixing parameter at low masses m<1 eV where the luminosity bound is weaker. In this regime the solar hidden photon flux has a sizable contribution of longitudinally polarized hidden photons of low energy which are invisible for current helioscopes. 
Javier Redondo