Abstract
Two-dimensional O2 physisorbed on graphite shows a structural and magnetic phase transition from the paramagnetic to the antiferromagnetic state. A high-resolution specific-heat measurement is presented which reveals that there are two successive phase transitions: first a three-state Potts transition, producing a distorted lattice being still paramagnetic, and second an elastically driven Ising transition from the paramagnetic to the antiferromagnetic state.