group more than 20% of the bursts are single pulsed, while there were no such bursts in the original *’long’* sample*.* Naturally, the bursts that loose the fine structure because of the noise are bursts with fewer original pulses. It is clear that short bursts have less pulses then long ones and therefore they are more “vulnerable” to this effect. Hence we conclude that at least one third of short bursts are highly variable ($ \delta
t_{min}/T\ll 1 $) and it is very likely that another third (those bursts with $ 0.1< \delta t_{min}/T < 1 $)is variable as well. We cannot tell whether the smooth structure ($ \delta
t_{min}/T = 1 $) seen in a third of the short bursts is intrinsic or whether it arises due to the noise.

While we are mostly interested in this analysis in the pulse width, it is worth mentioning that the amplitude of the variations is large. In most (61 out of 65) cases, when there is no overlap of nearby pulses the number of counts drops to less than half of the maximal counts. Thus the pulse we describe are not only statistically significant, they also correspond to a significant (factor 