  We construct a cosmological scalar-tensor-theory model in which the
Brans-Dicke type scalar $\Phi$ enters the effective (Jordan-frame) Hubble rate
as a simple modification of the Hubble rate of the $\Lambda$CDM model. This
allows us to quantify differences between the background dynamics of
scalar-tensor theories and general relativity (GR) in a transparent and
observationally testable manner in terms of one single parameter. Problems of
the mapping of the scalar-field degrees of freedom on an effective fluid
description in a GR context are discused. Data from supernovae, the
differential age of old galaxies and baryon acoustic oscillations are shown to
strongly limit potential deviations from the standard model.
