Standard deviation (σ_Ks) of the K_s-band light-curve extracted from the VVV fields d084 and d086. The fit of a theoretical relation between the K_s and the photometric accuracy is plotted in solid, green line. The variable candidates among the stars in the radius of the four studied clusters are shown. Big, red circles are stars having a variability amplitude higher than 5σ_Ks, while small, blue circles are stars with σ_Ks/σ_i ≥ 2.5, where σ_i is the internal scatter of the time-series.