r-band light curves of FBQ 0951+2635

The new optical records covering the last 17 yr were also used to trace the long-timescale evolution of the microlensing variability. Additionally, the new time delay interval and a relatively rich set of further observational constraints allowed us to discuss the mass structure of the main lensing galaxy at redshift 0.26.

This lens system is of particular interest because the external shear from secondary gravitational deflectors is relatively low, but the external convergence is one of the highest known. When modelling the galaxy as a singular power-law ellipsoid without hypotheses/priors on the power-law index, ellipticity and position angle, we demonstrated that its mass profile is close to isothermal, and there is good alignment between mass and near-IR light.