  We find evidence for long-range and short-range ($\zeta$ $=$ 70 \AA~at 4 K)
incommensurate magnetic order on the quasi-face-centered-cubic (FCC) lattices
of the monoclinic double perovskites La$_2$NaRuO$_6$ and La$_2$NaOsO$_6$
respectively. Incommensurate magnetic order on the FCC lattice has not been
predicted by mean field theory, but may arise via a delicate balance of
inequivalent nearest neighbour and next nearest neighbour exchange
interactions. In the Ru system with long-range order, inelastic neutron
scattering also reveals a spin gap $\Delta$ $\sim$ 2.75 meV. Magnetic
anisotropy is generally minimized in the more familiar octahedrally-coordinated
$3d^3$ systems, so the large gap observed for La$_2$NaRuO$_6$ may result from
the significantly enhanced value of spin-orbit coupling in this $4d^3$
material.
