Relation between S_flat and TESS magnitude for polars (pink) compared to the same relation for the M dwarf sample from Stelzer et al. (2022) (red) and a subsample of M dwarfs from Joseph et al. (2024) (blue). The TESS light curves of the latter were analysed by us (see Sect. 5). The lower envelope of the S_flat distribution (green dashed line) was obtained by fitting a second-order polynomial to the lower 25% quantile of the data. The two-hour rmsCDPP values for all objects in the three samples are presented for comparison (see Sect. 5 for details).