  Pseudoscalar particles like axion-like particles generically couple to two
photons, giving rise to the possibility of oscillations with photons in an
external magnetic field. These oscillations could lead to measurable imprints
in the electromagnetic spectrum of astrophysical sources if the coupling to
photons is strong enough. One possible signature is the presence of
irregularities in a limited energy range of the spectrum. In this study,
anomalous irregularities are searched for in the spectrum of the bright
extragalactic TeV emitter PKS 2155-304 as measured by H.E.S.S. to put
constraints on the coupling of light spin 0 particles to photons.
