  Feebly interacting particles with masses with O(10-100) MeV can be copiously
produced by core-collapse supernovae (SNe). In this paper we consider the case
of MeV-ish sterile neutrinos and dark photons mixed with ordinary neutrinos and
photons, respectively. Furthermore, both sterile neutrinos and dark photons may
decay into positrons on their route to Earth. Such positrons would annihilate
with electrons in the Galactic medium and contribute to the photon flux in the
511 keV line. Using the SPI (SPectrometer on INTEGRAL) observation of this line
improves the bounds on the mixing parameters for these particles by several
orders of magnitude below what is already excluded by the SN 1987A energy-loss
argument.
