Heavy quark spin structure in Zb resonances

We discuss the heavy quark spin structure of the recently observed twin resonances Z(b)(10610) and Z(b)(10650), assuming that these are mostly of a molecular type, i.e. that their internal dynamics is dominated by the coupling to meson pairs B*(B) over bar - B (B) over bar* and B*(B) over bar*. We find that the state of the b (b) over bar pair within the Z(b)(10610) and Z(b)(10650) resonances is a mixture of a spin-triplet and a spin-singlet of equal amplitude and with the phase orthogonal between the two resonances. Such a structure gives rise to specific relations between observable amplitudes that are in agreement with the data obtained recently by Belle. We also briefly discuss possible properties of the isotopically singlet counterparts of the newly found resonances, and also of their C (G) parity opposites that likely exist in the same mass range near the open B flavor threshold.