  Ground-based photometric surveys have led to the discovery of six transiting
exoplanets, five of which were detected by the OGLE survey. The FLAMES
multi-object spectrograph on the VLT has permitted a very efficient follow-up
of the OGLE transit candidates, characterising not only the 5 planets but also
more than 50 systems producing similar photometric signatures -- mainly
eclipsing binaires. The presence of these ubiquitous "impostors" is a challenge
for transit surveys. Another difficulty is the presence of red noise in the
photometry, which implies a much lower sensitivity to transiting planets than
usually assumed. We outline a method to estimate how the red noise will affect
the expected yield of photometric transit searches.
