  In this short paper, we study the photometric precision of stellar light
curves obtained by the CoRoT satellite in its planet finding channel, with a
particular emphasis on the timescales characteristic of planetary transits.
Together with other articles in the same issue of this journal, it forms an
attempt to provide the building blocks for a statistical interpretation of the
CoRoT planet and eclipsing binary catch to date.
  After pre-processing the light curves so as to minimise long-term variations
and outliers, we measure the scatter of the light curves in the first three
CoRoT runs lasting more than 1 month, using an iterative non-linear filter to
isolate signal on the timescales of interest. The bevhaiour of the noise on 2h
timescales is well-described a power-law with index 0.25 in R-magnitude,
ranging from 0.1mmag at R=11.5 to 1mmag at R=16, which is close to the
pre-launch specification, though still a factor 2-3 above the photon noise due
to residual jitter noise and hot pixel events. There is evidence for a slight
degradation of the performance over time. We find clear evidence for enhanced
variability on hours timescales (at the level of 0.5 mmag) in stars identified
as likely giants from their R-magnitude and B-V colour, which represent
approximately 60 and 20% of the observed population in the direction of Aquila
and Monoceros respectively. On the other hand, median correlated noise levels
over 2h for dwarf stars are extremely low, reaching 0.05mmag at the bright end.
