by the minima between neighboring peaks if the counts do not drop below quarter of the maximum (see the Appendix for a detailed definition and for a discussion of the algorithm). An elementary pulse in the observed light curve must not necessarily correspond directly to an elementary emission event in the source. A single pulse in the light curve could, in principle, be composed of numerous emission events. In this case the emission process is even more variable then the observed light curve and the results we obtain here should be considered only as upper limits to the intrinsic variability of the sources.

We find that most short bursts (although not all of them) are highly variable ($ \delta t_{min}/T\ll 1 $) **.** When analyzing the high resolution data of long burst we find that the shortest time scales seen in long bursts are similar to those in short bursts. This result is limited to the highest resolution in which we analyzed the long bursts - 5ms. Such time scales were already observed in at least one long burst (Lee, Bloom & Petrosian 2000). We show here that these time scales are common.

In section 2 we describe the data 