Rotation period distribution of Gaia DR2 data for BY Dra stars (P is given in day units). A 3 d binning P histogram is shown in the left panel, together with the Gaussian kernel density estimate with a 3 d width and 3-sigma truncation. The P – ecliptic latitude scatter plot (right panel) shows the higher number of period measurements at ± 45° ecliptic latitudes, particularly regarding longer periods. The colour code (rainbow) is an indication of the relative density of data points (red for lower density, and blue for higher density). Both panels give an indication of the moderate degree of aliasing, the most evident being at ≈23.5, 25.3, and 31.6 d.