  The behavior of the so-called weak moment antiferromagnetic states, observed
in the heavy-fermion superconductors UPt$_3$ and URu$_2$Si$_2$, is discussed in
view of recent $\mu$SR results obtained as function of control parameters like
chemical substitution and external pressure. In UPt$_3$, the Pd substitution
for Pt reveals the dynamical character of the weak moment order. On the other
hand, $\mu$SR measurements performed on samples in which Th substitutes U
suggest that crystallographic disorder on the magnetic sites deeply affects the
fluctuation timescale. In URu$_2$Si$_2$, a phase separation between the
so-called hidden order state, present at ambient pressure, and an
antiferromagnetic state, occurring under pressure, is observed. In view of the
pressure-temperature phase diagram obtained by $\mu$SR, it is deduced that the
respective order parameters have different symmetries.
