(B − Ṁ) diagram for the polars with values from Table 2. The sources are shown by filled symbols with increasing sizes and colours corresponding to their mass in the range 0.3–0.5 (blue), 0.5–0.7 (green), 0.7–0.9 (red), and 0.9–1.2 M_⊙ (black). Sources with no mass determination are shown with open symbols and assumed at 1 M_⊙. The 5 polars with known optical QPOs are shown in italics. The lines mark the limit of the QPO regime corresponding to ϵ_s = 1 (see text), assuming a representative column cross-section of 10¹⁴ cm² and for different white dwarf mass values shown by labels and with the above colour convention. QPOs are expected above the associated line.