The “trial curves”, i.e., a set of artificial 4-season long light curves of a quad lens, used in place of real observations for the self-consistency check performed in Sect. 8. These curves are drawn from a generative model that closely mimics COSMOGRAIL observations. Note the prominent microlensing on large time scales, but also the presence of obviously extrinsic variability on scales of weeks (e.g., middle of third season of curve C). The true time delays are Δt_AB =  −5.0, Δt_AC =  −20.0, and Δt_AD =  −70.0 days. In this figure the curves are shifted according to these delays.