4 Summary

Even the recent high localization accuracy of GRBs cannot exclude positionally coinciding OTs as just random unrelated coincidences. The exact background rate of unrelated triggers such as variable stars and extragalactic objects is unknown but may be rather high. The expected rate of faint (lim mag B 22.6) variable AGNs is 1 in a $5 \times 5$ arcmin box. The statistics of large-amplitude (>0.5 mag) variable AGNs is unknown. The rate of faint (below mag 20) variable stars is unknown. For brighter variable stars, the expected rate is 0.03 - 0.5 (depending on galactic latitude) in a $5 \times 5$ arcmin box (lim mag 20). The expected rate of faint (lim mag R 23) SNe is 0.4 in a $5 \times 5$ arcmin box. The estimated total number of optically variable sources of astrophysical origin (SNe+AGNs+VS) is $\sim 1-2$ in a $5 \times 5$ arcmin box, lim mag 23. It is obvious that we need a better statistics of faint variable optical objects.

Acknowledgements

The work has been supported by the Projects of the Czech Ministry for Education and Youth, Nos. ES02 and ES036.