Melting of the first monolayer of Ar adsorbed on the basal face of graphite

We have measured adsorption isotherms of Ar on graphite from 67 to 77 K in the range of coverages at which the monolayer freezes. The transition appears continuous and shows some similarity with a λ transition. The large increase of the isothermal compressibility at freezing seems to indicate that it cannot be accounted for by the Kosterlitz-Thouless-Halperin-Nelson-Young theory.