  We address what seemed to be a contradiction between the lanthanide series
REBa$_2$Cu$_3$O$_y$ (RE123) and the charge-compensated series
(Ca$_{x}$La$_{1-x}$)(Ba$_{1.75-x}$La$_{0.25+x} $)Cu$_{3}$O$_{y}$ (CLBLCO)
regarding the superexchange ($J$) dependence of the maximum superconductivity
(SC) critical temperature $T_c^{max}(J)$; RE and $x$ are implicit variables.
This is done by measuring the N\'{e}el temperature and the temperature
dependence of the magnetic order parameter for RE=Nd, Sm, Eu, Gd, Dy, Yb, Y,
and for Y(BaSr)Cu$_3$O$_y$, at various very light dopings. The doping is
determined by thermopower, and the magnetic properties by muon spin rotation.
We find that the normalized-temperature dependence of the order parameter is
identical for all RE123 in the undoped limit (with the exception of Gd123)
implying identical out-of-plane magnetic coupling. The extrapolation of $T_N$
to zero doping suggests that, despite the variations in ionic radii, $J$ varies
too weakly in this system to test the relation between SC and magnetism. This
stands in contrast to CLBLCO where both $T_c^{max}$ and $T_N^{max}$ vary
considerably in the undoped limit, and a positive correlation between the two
quantities was observed.
