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The helicity-dependent cross sections for the photoproduction of $\pi^0\eta$pairs have been measured for the first time. The experiment was performed atthe tagged photon facility of the Mainz MAMI accelerator with the combinedCrystal Ball - TAPS calorimeter. The experiment used a polarized deuteratedbutanol target and a circularly polarized photon beam. This arrangement allowedthe $\sigma_{1/2}$ (photon and target spin antiparallel) and $\sigma_{3/2}$(parallel spins) components to be measured for quasi-free production of$\pi^0\eta$ pairs off protons and neutrons. The main finding is that the twohelicity components contribute identically, within uncertainties, for bothparticipant protons and neutrons. The absolute couplings for protons andneutrons are also identical. This means that nucleon resonances contributing tothis reaction in the investigated energy range have almost equalelectromagnetic helicity couplings, $A_{1/2}^{n,p}$ and $A_{3/2}^{n,p}$.Identical couplings for protons and neutrons are typical for $\Delta$resonances and identical $A_{1/2}$ and $A_{3/2}$ components are only possiblefor $J\geq 3/2$ states, which constrains possible contributions of nucleonresonances.

First measurement of helicity-dependent cross sections in pi0-eta photoproduction from quasi-free nucleons