  Classical Heisenberg antiferromagnets on two-dimensional kagome and
three-dimensional hyperkagome lattices are investigated by Monte Carlo
simulations. For both models the symmetry-breaking states at low temperatures
are described by non-zero octupole moments or third-rank spin tensor order
parameters. In the case of the two-dimensional kagome antiferromagnet, a sharp
crossover into a coplanar state takes place at T_k ~ 0.004J, which we attribute
to proliferation of fractional vortices. The three-dimensional model exhibits a
first-order transition at T_c ~ 0.002J into a phase with critical spin
correlations, which possesses a long-range order of octupole moments.
