  Recently it has been noticed that the Fermi-LAT data of gamma-rays from some
galactic pulsars and supernova remnants reveal spectral modulations that might
be explained by the conversion of photons to ALPs (axion-like particles)
induced by the conventional ALP coupling to photon in the presence of galactic
magnetic fields. However the corresponding ALP mass and coupling are in a
severe tension with the observational constraints from CAST, SN1987A, and other
gamma-ray observations. Motivated by this, we examine an alternative
possibility that those spectral modulations are explained by other type of ALP
coupling involving both the ordinary photon and a massless dark photon, when
nonzero background dark photon gauge fields are assumed. We find that our
scheme results in oscillations among the photon, ALP, and dark photon, which
can explain the gamma-ray spectral modulations of galactic pulsars or supernova
remnants, while satisfying the known observational constraints.
