structure of short ($ T_{90}<2sec $) bursts. This is not surprising. Short bursts are much harder to analyze because of their significantly lower signal to noise ratios. We developed a new algorithm that is sensitive enough to identify pulses in short bursts. Using this algorithm we search here for subpulses in short bursts and determine their duration ($ \delta t $). This enables us to set upper limits on the shortest time scale seen in the bursts to set limits on the variability of short bursts.

Sari & Piran (1997) and Fenimore, Madras, & Nayakshin (1996) have shown that angular spreading would smooth any variability produced by external shocks (unless the GRB production is very inefficient with the efficiency of the order of $ (\delta
t_{min}/T) $ defined below (Sari & Piran, 1997)). The critical parameter in this analysis is the ratio between the shortest time scale, on which the burst varies significantly, and the longest time scale in the burst. This motivates us to focus here on the ratio $ \delta t_{min}/T $, where $\delta t_{min}$ is the minimal observed duration of an individual pulse and $T$ is the duration of the burst. High variability means low $ \delta
