  The HARPS spectrograph is showing an extreme stability close to the m s-1
level over more than ten years of data. However the radial velocities of some
stars are contaminated by a spurious one-year signal with an amplitude that can
be as high as a few m s-1 . This signal is in opposition of phase with the
revolution of Earth around the Sun, and can be explained by the deformation of
spectral lines crossing block stitchings of the CCD when the spectrum of an
observed star is alternatively blue- and red-shifted due to the motion of Earth
around the Sun. This annual perturbation can be supress by either removing
those affected spectral lines from the correlation mask used by the cross
correlation technique to derive precise radial velocities, or by simply fitting
a yearly sinusoid to the RV data. This is mandatory if we want to detect
long-period low-amplitude signals in the HARPS radial velocities of quiet
solar-type stars.
