10-08E

We consider the quantum Heisenberg antiferromagnet in a magnetic field on two one-dimensional lattices containing equilateral triangles (a chain of corner-sharing double tetrahedra and a frustrated three-leg ladder) which support localized-magnon states. By mapping of the localized-magnon degrees of freedom on a classical lattice gas we obtain high-field thermodynamic quantities of the models at low temperatures. This paper was presented at the 14th Czech and Slovak Conference on Magnetism CSMAG'10 (6--9 July 2010, Ko\v{s}ice, Slovakia).