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In analogy to curve complexes used to study mapping class groups of surfaces, the free factor graph of a free group $$F_n$$ has recently turned to be fruitful in the study of Out($$F_n$$). It is Gromov hyperbolic, as was proved by M. Bestvina and M. Feighn [Adv. Math. 256, 104–155 (2014; Zbl 1348.20028)], and the action of an automorphism of $$F_n$$ is loxodromic if and only if it is fully irreducible. Its Gromov boundary was described by M. Bestvina and P. Reynolds [Duke Math. J. 164, No. 11, 2213–2251 (2015; Zbl 1337.20040)] and U. Hamenstädt [“The boundary of the free splitting graph and the free factor graph”, Preprint, arXiv:1211.1630] as the set of equivalence classes of arational trees.