  When imaged at high-resolution, many proto-planetary discs show gaps and
rings in their dust sub-mm continuum emission profile. These structures are
widely considered to originate from local maxima in the gas pressure profile.
The properties of the underlying gas structures are however unknown. In this
paper we present a method to measure the dust-gas coupling $\alpha/St$ and the
width of the gas pressure bumps affecting the dust distribution, applying
high-precision techniques to extract the gas rotation curve from emission lines
data-cubes. As a proof-of-concept, we then apply the method to two discs with
prominent sub-structure, HD163296 and AS 209. We find that in all cases the gas
structures are larger than in the dust, confirming that the rings are pressure
traps. Although the grains are sufficiently decoupled from the gas to be
radially concentrated, we find that the degree of coupling of the dust is
relatively good ($\alpha/St \sim 0.1$). We can therefore reject scenarios in
which the disc turbulence is very low and the dust has grown significantly. If
we further assume that the dust grain sizes are set by turbulent fragmentation,
we find high values of the $\alpha$ turbulent parameter ($\alpha \sim
10^{-2}$). Alternatively, solutions with smaller turbulence are still
compatible with our analysis if another process is limiting grain growth. For
HD163296, recent measurements of the disc mass suggest that this is the case if
the grain size is 1mm. Future constraints on the dust spectral indices will
help to discriminate between the two alternatives.
