Process of detection and selection of candidate RR Lyrae is illustrated using B336 as an example. Upper panel: the standard deviation of the five individual measures of F606W is plotted as a function of the average F606W. The candidates RR Lyrae were selected among stars with 24.9 ≤ F606W ≤ 25.4, as stars with rms deviations larger then 3σ (open stars). The continuous line indicates the 3σ level and the dotted lines the 2 and 1σ levels. Lower panels: light curves of the candidates RR Lyrae detected in the upper panel and labeled with 1, 2, and 3, taken as examples. Candidates 1 and 2 show coherent patterns of variations at all sampled epochs, compatible with genuine RR Lyrae variables. Candidate 3 is a classical case of spurious detection: F606W is constant at all epochs except one, likely corresponding to the hit of a cosmic ray over the PSF area of the star in the individual image at that epoch.