  We give exact integral expressions of the third cluster or virial
coefficients of binary mixtures of ideal Bose or Fermi gases, with interspecies
interactions of zero range and infinite s-wave scattering length. In general
the result depends on three-body parameters Rt appearing in three-body contact
conditions, because an Efimov effect is present or because the mixture is in a
preefimovian regime with a mass ratio close to an Efimov effect threshold. We
give a new, exact integral expression of Rt for the microscopic narrow Feshbach
resonance model. A divergence of Rt in the preefimovian regime at a scaling
exponent s = 1/2 is predicted and physically discussed. The analytical results
are applied to typical species used in cold atom experiments.

Quantum Gases