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Observational constraints on dust properties in protoplanetary disks are keyto better understanding disks' evolution. We continue our exploration of theprotoplanetary disk around AB Aur by characterizing its dust properties. Wepresent ALMA observations at 2.2 mm and VLA observations at 6.8 mm. Togetherwith previous ALMA and NOEMA observations at 0.87 and 1.1 mm, theseobservations are used to compute global spectral index profiles as well asspectral index maps. On the interpretation side, we present the results of asimple isothermal slab model to help constrain dust properties along the ringof continuum emission. We also present results of dust radiative transfercalculations based on a disc-planet hydrodynamical simulation to explain howthe azimuthal contrast ratio of the ring emission varies with millimeterwavelength. The spectral energy distribution and the radial profiles of thespectral index indicate that the radiation from the compact source towards thecenter is not dominated by dust thermal emission, but most likely by free-freeemission originating in the radio jet: it constitutes 93% of the emission at6.8 mm, and 37% at 0.87 mm. The protoplanetary disk has a typical spectralindex of 2.3. We estimate a dust disk mass of 8$\rm \times 10^{-5}$ M$\rm_{\odot}$ which, assuming a mean gas-to-dust ratio of 40, gives a total diskmass of 3.2$\rm \times 10^{-3}$ M$\rm _{\odot}$. The azimuthal contrast ratioof the ring outside the millimeter cavity is smaller at 2.2 mm than at 1.1 mm,in agreement with previous findings. The VLA image shows several knots of$5\sigma$ emission all along the ring, which are consistent with the ringemission being nearly axisymmetric at that wavelength. The decrease in theazimuthal contrast ratio of the ring emission from 0.87 to 6.8 mm can beexplained by a dust-losing decaying vortex at the outer edge of a planet gap.

AB Aur, a Rosetta stone for studies of planet formation (III): continuum observations at 2 and 7 mm