  During a serendipitous observation of the BeppoSAX Wide Field Cameras, a very
long Type I X-ray burst was observed from the low mass X-ray binary Serpens
X-1. The burst lasted for approximately 4 hours and had an exponential decay
time of 69+/-2 min (2-28 keV). The bolometric peak-luminosity is
(1.6+/-0.2)x10^38 erg/s and the fluence (7.3+/-1.4)x10^41 erg. The first
'normal' Type I burst was observed 34 days after the superburst. This is in
rough agreement with recent predictions for unstable carbon burning in a heavy
element ocean.
