rms magnitude deviation as a function of the mean magnitude in the filters V, R, and I. Known variables are labelled. V1 (dark blue point) and V2 (green point) are two known RR Lyrae stars in the field of the cluster. Red circles correspond to two newly identified SX Phe stars and one candidate SX Phe. The variable V4 is saturated in our R reference image and the newly identified variable V5 is saturated in the R and I reference images. Hence these stars are not shown in the corresponding plots. The cyan points correspond to the blue stragglers identified by Cote et al. (1991).