Heavy quark spin structure in hidden charm molecules

Recently observed heavy quark exotic XYZ mesons stimulated heated discussions about their structures. In the molecule picture, we discuss the spin of the heavy quark pair in an S-wave meson-antimeson state, whose value is either 1 or 0. We find two rules that the spin is only 1 in the heavy quark limit. From the conservation of the spin, one may analyze the charmonium products from strong decays. The selection rules give constraints on the products.