We study a sample of bright long BATSE GRB light curves in the 200 s before the detection of the GRB prompt emission .
We find that in a sizable fraction of cases ( \sim 20 \% ) there is evidence of emission above the background coming from the same direction of the GRB .
This emission is characterised by a softer spectrum with respect to the main one and contains a small fraction ( 0.1 - 1 \% ) of the total event counts .
The precursors have typical delays of several tens of seconds extending ( in few cases ) up to 200 seconds ( the limit of the investigated period ) .
Their spectra are typically non-thermal power-law but for a few cases .
Such long delays and the non-thermal origin of their spectra are hard to reconcile with any model for the progenitor .