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 \pm 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 .