  The detrending algorithms which are widely used to reduce the impact of
stellar variability on space-based transit surveys are ill-suited for
estimating the parameters of confirmed planets, as they unavoidably alter the
transit signal. We present a post-detection detrending algorithm, which filters
out signal on other timescales than the period of the transit while preserving
the transit signal.
  We compare the performance of this new filter to a well-established
pre-detection detrending algorithm, by applying both to a set of 20 simulated
light curves containing planetary transits, stellar variability, and
instrumental noise as expected for the CoRoT space mission, and performing
analytic fits to the transits. Compared to the pre-detection benchmark, the new
post-detection filter systematically yields significantly reduced errors
(median reduction in relative error over our sample of about 40%) on the
planet-to-star radius ratio, system scale and impact parameter. This is
particularly important for active stars, where errors induced by variability
can otherwise dominate the final error budget on the planet parameters.
  Aside from improving planet parameter estimates, the new filter preserves all
signal at the orbital period of the planet, and thus could also be used to
search for light reflected by the planet.
