Distribution of the photometric periods of the variable UHE and He II line problem white dwarfs (blue), and the period distribution of only the UHE white dwarfs (purple). On the left their period distribution is compared to the orbital period distribution of PCE CSPNe (light green; the bold teal line indicates the period distribution of binary CSPNe that show a reflection effect) and white dwarfs plus main sequence binaries (light yellow). Left panel: a comparison with the rotational periods of normal white dwarfs (light green with dashed contours) and magnetic white dwarfs (bold yellow lines). The median period and standard deviation of each sample is indicated.