  We report the detection with the BeppoSAX Wide Field Cameras of a superburst
from 4U 1254-690. The superburst is preceded by a normal type-I X-ray burst,
has a decay time that is the longest of all eight superbursts detected so far
and a peak luminosity that is the lowest. Like for the other seven superbursts,
the origin is a well-known type-I X-ray burster with a persistent luminosity
level close to one tenth of the Eddington limit. Based on WFC data of all
persistently bright X-ray bursters, the average rate of superbursts is
0.51+/-0.25 per year per persistently bright X-ray burster. Some systems may
have higher superburst rates. For all superbursters, we present evidence for a
pure helium layer which is burnt in an unstable as well as a stable manner.
