Observed V² (after flux-ratio and flux-bias corrections; see Sect. 2.1) plotted against observing time. Gray dots are individual measurements for blocks of 5 s each. These blocks are averaged into scans over a few to several minutes each. These scans are indicated by various symbols with error bars corresponding to statistical errors, calculated as standard deviations of the blocks within each scan. Gray circles are the estimated system V² at the time of target observations (plotted slightly shifted in time for clarity).