Left panel: a) fitting function to the standard deviation versus the mean differential magnitude for the reference star sample, using aperture radius of 1, 3, 5, and 10 pixel. The top panel shows the fits for the H band, bottom panel for the K_s band. The black line represents the aperture radius considered for the photometry in the two bands. Right panel: b) standard deviation versus the mean differential magnitude for the reference star sample and the targets in the H band (top) and the K_s band (bottom). Candidate variables are marked with black circles.