  The hypothetical massive dark photon ($\gamma'$) which has kinetic mixing
with the SM photon can decay electromagnetically to $e^+e^-$ pairs if its mass
$m$ exceeds $2m_e$ and otherwise into three SM photons. These decays yield
cosmological and supernovae associated signatures. We briefly discuss these
signatures, particularly in connection with the supernova SN1987A and delineate
the extra constraints that may then arise on the mass and mixing parameter of
the dark photon. In particular, we find that for dark photon mass $m_{\gamma'}$
in the 5-20 MeV range, arguments based on supernova 1987A observations lead to
a bound on $\epsilon$ which is about 300 times stronger than the presently
existing bounds based on energy loss arguments.
