Upper limits on polar (0.1–5 Hz) oscillation amplitudes in the X-ray (0.5–10 keV) flux as a function of magnetic field strength. Only sources with upper limits (<50%) are shown. Sources with known optical QPOs are denoted by an asterisk. Also displayed are the QPO amplitudes predicted by representative numerical simulations of the shock instability (shown by symbols). Curves are polynomial fits through the individual measurements and are shown for a typical WD mass of 0.8 M_⊙, a column cross-section of 10¹⁴ cm² and for different values of the specific accretion rate (dashed curve ṁ = 1.0, full curve ṁ = 10 and dotted curve ṁ = 100 g cm^-2 s^-1).