Suppose that G is a group, H and K are proper isomorphic central subgroups of G, and G is an HNN-extension of G with the associated subgroups H and K. We prove necessary and sufficient conditions for G to be residually a C-group, where C is a class of groups closed under taking subgroups, extensions, homomorphic images, and Cartesian products of the form y∈Y X y , where X, Y ∈ C and X y is an isomorphic copy of X for each y ∈ Y .