Comparison of the observed spectrum (black dots) at the location of the λ7800 Å of Rb I line for the M dwarfs G244-77 (non-active, left panel) and OT Ser (active, right panel) studied in Paper I, with synthetic spectra (continuous and dashed lines) computed with different average magnetic field intensities in the line of sight and Rb abundances (as labelled). Only the Rb I line is included in the spectral synthesis (see text for details).