Hardness ratios as a function of the count rates, in the 0.3−10.0 keV energy band, and in units cts s^-1, for four clearly variable sources (for EPIC-pn and for 1 ks bins). The hardness ratios are defined as the ratios between count rates in the 1.0–10.0 keV and 0.3–1.0 keV energy bands. The correlation coefficients amount to 40% for HD 36960, −9% for HD 52089, 81% for HD 79351, and 5% for HD 158926.