  To detect brown dwarfs in the dark galactic halo through gravitational
lensing, experiments follow the luminosity of millions of stars to observe a
few lensing events par year. The luminosity of a star too faint to be
continuously followed can be temporarily increased above the detection limit by
a lensing. The detection of these invisible stars would increase the number of
events by a factor 2 to 7, and moreover their presence would confirm the
lensing interpretation of events due to continuously monitored stars.
