  We show that all long nulls in PSR B0818-13 are trains of rapidly alternating
nulls and pulses (each shorter than one pulse period). Sometimes only the nulls
coincide with our pulse window, resulting in one of the apparently long nulls
seen occasionally. We show these are seen as often as expected if during such a
train the probability for nulls is 1.2 times less than for pulses.
  During nulls, the subpulse drift-speed appears to increase. We assume that
the carousel of sparks that possibly underlies the subpulses actually slows
down, as it does in similar pulsars like PSR B0809+74, and conclude that the
subpulse-drift in this pulsar must be aliased. The carousel must then rotate in
30 seconds or less, making it the fastest found to date.
