  In the heavy quark limit, we discuss the spin of the charm-anticharm pair
$J_{c\bar{c}}$ in an S-wave meson-antimeson molecule or resonance. One finds
two cases that $J_{c\bar{c}}$ cannot be 0: (a) $J=|j_2-j_4|-1$ or $J=j_2+j_4+1$
where $J$ is the total spin of the system and $j_2$ ($j_4$) is the total
angular momentum of the light degree of freedom in a charmed meson (antimeson);
(b) $J^C=1^+,2^-,3^+,...$, if the two different mesons belong to the same
doublet. The decays to spin-singlet charmonium states are suppressed when one
of the two conditions is satisfied. We discuss constrained decay properties for
selected systems.
