3 Discussion

Let us briefly discuss about the probability to observe repeating GRBs. This eventuality has been debated by different authors in the framework of several (more or less simple) theoretical models. For example, to use a single isothermal sphere, the probability of a single repetition is 0.05-0.4% (Mao 1992). Using the estimate of the observation probability of QSO gravlensing of 1% (Wu 1996), we obtain that $0.1\%-0.3\%$ of cosmological GRBs can be repeated twice.

Finally we stress that the possibility to analyze the problem of obtaining GRBs light curves by means of gravlensing is the result of the detection of the optical transient with high spatial accuracy.

Acknowledgements

This investigation was supported by the Russian Fund of Fundamental Researches (grant 98-02-17570), by the Educational-Scientific centre "Cosmion", by the Italian Ministry of Foreign Affairs, and by the University of Bologna (Funds for selected research topics).