  We revisit a scenario in which the cosmological constant is cancelled by the
potential energy of a slowly evolving scalar field, or "cosmon". The cosmon's
evolution is tied to the cosmological constant by a feedback mechanism. This
feedback is achieved by an unconventional coupling of the cosmon field to the
Ricci curvature scalar. The solutions show that the effective cosmological
constant evolves approximately as $t^{-2}$ and remains always of the same order
as the density of ordinary matter and radiation. Newton's constant varies on
cosmological time scales, with $\dot{G}_N/G_N \ll 1/t$. $G_N$ could have been
somewhat different, and possibly smaller, at the time of Big Bang
nucleosynthesis.
