Recently structures in invariant mass distributions and excitation energy spectra have been attributed to triangular singularities as discussed in e.g.,[1,2] and in the review by Guo et al. . These singularities emerge under specific kinematic conditions when new reaction channels open up. It will be shown that a triangular singularity associated with the opening of the γ p →p a0 → p π0 η
channel may explain a structure in the Mpη invariant mass distribution near 1700 MeV/c2 observed in the γ p → p π0 η reaction .
 G. D. Alexeev et al., The COMPASS Collaboration, Phys. Rev. Lett
127, 082501 (2021)
 M. Mikhasenko, B. Ketzer and A. Sarantsev, Phys. Rev. D 91, 094015
 F. K. Guo et al., Rev. Mod. Phys. D 90, 015004 (2018).
 V. Metag et al., EPJA 57 (2021) 325.
Supported by DFG through SFB/TR16.