  Sys-Rem (Tamuz, Mazeh & Zucker 2005) is a detrending algorithm designed to
remove systematic effects in a large set of lightcurves obtained by a
photometric survey. The algorithm works without any prior knowledge of the
effects, as long as they appear in many stars of the sample. This paper
presents the basic principles of Sys-Rem and discusses a parameterization used
to determine the number of effects removed. We assess the performance of
Sys-Rem on simulated transits injected into WHAT survey data. This test is
proposed as a general scheme to assess the effectiveness of detrending
algorithms. Application of Sys-Rem to the OGLE dataset demonstrates the power
of the algorithm. We offer a coded implementation of Sys-Rem to the community.
