  We present optical photometry of the cataclysmic variable LAMOST
J024048.51+195226.9 taken with the high-speed, five-band CCD camera HiPERCAM on
the 10.4 m Gran Telescopio Canarias (GTC). We detect pulsations originating
from the spin of its white dwarf, finding a spin period of 24.9328(38)s. The
pulse amplitude is of the order of 0.2% in the g-band, below the detection
limits of previous searches. This detection establishes LAMOST
J024048.51+195226.9 as only the second white dwarf magnetic propeller system, a
twin of its long-known predecessor, AE Aquarii. At 24.93s, the white dwarf in
LAMOST J024048.51+195226.9 has the shortest known spin period of any
cataclysmic variable star. The white dwarf must have a mass of at least 0.7MSun
to sustain so short a period. The observed faintest u-band magnitude sets an
upper limit on the white dwarf's temperature of ~25000K. The pulsation
amplitudes measured in the five HiPERCAM filters are consistent with an
accretion spot of ~30000K covering ~2% of the white dwarf's visible area,
although spots that are hot and smaller, or cooler and larger cannot be ruled
out.
