  We report on the observation of a dc Stark frequency shift at the $10^{-13}$
level by comparing two strontium optical lattice clocks. This frequency shift
arises from the presence of electric charges trapped on dielectric surfaces
placed under vacuum close to the atomic sample. We show that these charges can
be eliminated by shining UV light on the dielectric surfaces, and characterize
the residual dc Stark frequency shift on the clock transition at the $10^{-18}$
level by applying an external electric field. This study shows that the dc
Stark shift can play an important role in the accuracy budget of lattice
clocks, and should be duly taken into account.
