  One of the clear predictions of string theory is the presence of a dynamical
scalar partner of the spin-2 graviton, known as the dilaton. This will violate
the Einstein Equivalence Principle, leading to a plethora of possibly
observable consequences which is a cosmological context include dynamical dark
energy and spacetime variations of nature's fundamental constants. The runaway
dilaton scenario of Damour, Piazza and Veneziano is a particularly interesting
class of string theory inspired models which can in principle reconcile a
massless dilaton with experimental data. Here we use the latest background
cosmology observations, astrophysical and laboratory tests of the stability of
the fine-structure constant and local tests of the Weak Equivalence Principle
to provide updated constraints on this scenario, under various simplifying
assumptions. Overall we find consistency with the standard $\Lambda$CDM
paradigm, and we improve the existing constraints on the coupling of the
dilaton to baryonic matter by a factor of six, and to the dark sector by a
factor of two. At the one sigma level the current data already excludes dark
sector couplings of order unity, which would be their natural value.
