“…Recently, Allenby, Kim, Tang, Chai and Choi in several papers [1,2,7,13,16], have made important contributions to the study of the residual finiteness of outer automorphism groups of various groups. In [1], Allenby, Kim and Tang improved on Grossman's results by proving that the outer automorphism groups of generalized free products of two free groups amalgamating a maximal cyclic subgroup are residually finite.…”