  Starting from the hypothesis of a second order transition we have studied
modifications of the original Heisenberg antiferromagnet on a stacked
triangular lattice (STA-model) by the Monte Carlo technique. The change is a
local constraint restricting the spins at the corners of selected triangles to
add up to zero without stopping them from moving freely (STAR-model). We have
studied also the closely related dihedral and trihedral models which can be
classified as Stiefel models. We have found indications of a first order
transition for all three modified models instead of a universal critical
behavior. This is in accordance with the renormalization group investigations
but disagrees with the Monte Carlo simulations of the original STA-model
favoring a new universality class. For the corresponding x-y antiferromagnet
studied before, the second order nature of the transition could also not be
confirmed.
