Effective range expansion for narrow near-threshold resonances.

We discuss some general features of the effective range expansion, the content of its parameters with respect to the nature of the pertinent near-threshold states and the necessary modifications in the presence of coupled channels, isospin violations and unstable constituents. The properties of the $\chi_{c1}(3872)$, also known as the $X(3872)$, as well as the $T_{cc}^+$ states are analysed as illustrative examples.