Distribution of the S_Q statistic (see text) applied to the V-band light curves of all stars in our sample (black filled circles). A blue horizontal line marks the threshold of S_Q = 0.3 below which we inspected the light curves for periodic variability. The locations of the confirmed variables in our sample are marked with red filled circles; V7 is not shown as it does not appear to be periodic, but rather shows long-term variability.