  We show the first SPHERE/IRDIS and IFS data of the CO-rich debris disc around
HD 131488. We use N-body simulations to model both the scattered light images
and the SED of the disc in a self-consistent way. We apply the
Henyey-Greenstein approximation, Mie theory, and the Discrete Dipole
Approximation to model the emission of individual dust grains. Our study shows
that only when gas drag is taken into account can we find a model that is
consistent with scattered light as well as thermal emission data of the disc.
The models suggest a gas surface density of $2\times10^{-5}\,M_\oplus/$au$^2$
which is in agreement with estimates from ALMA observations. Thus, our
modelling procedure allows us to roughly constrain the expected amount of gas
in a debris disc without actual gas measurements. We also show that the shallow
size distribution of the dust leads to a significant contribution of large
particles to the overall amount of scattered light. The scattering phase
function indicates a dust porosity of $\sim0.2\ldots 0.6$ which is in agreement
with a pebble pile scenario for planetesimal growth.
