K_s band light-curves of known QSOs with respect to their mean magnitude. From top to bottom and left to right they follow the order in Table 2. Points represent the simple average of observations obtained within the same night/epoch and error bars correspond to the error of the mean. A continuous line connects the data points while a dashed line represents a linear fit through them. Day 0 corresponds to the first observation in the VMC data. Single epoch data are included for completeness.