Pseudo-equivalent width(He I) as a function of pEW(Ca II IRT₁) for six representative M4.0 stars. Measurements are designated by the symbols, while linear fits to the measurements are given by the solid lines. The CARMENES identification numbers of the stars are given in the legend together with the respective Pearson’s correlation coefficient. The outlier caused by a flare for J02519+224/RBS 365 is denoted with an additional circle.