  We consider the hypothesis that dark energy and dark matter are the two faces
of a single dark component, a unified dark matter (UDM) that we assume can be
modeled by the affine equation of state (EoS) $P= p_0 +\alpha \rho$, resulting
in an {\it effective cosmological constant} $\rho_\Lambda=-p_0/(1+\alpha)$. The
affine EoS arises from the simple assumption that the speed of sound is
constant; it may be seen as an approximation to an unknown barotropic EoS
$P=P(\rho)$, and may as well represent the tracking solution for the dynamics
of a scalar field with appropriate potential. Furthermore, in principle the
affine EoS allows the UDM to be phantom. We constrain the parameters of the
model, $\alpha$ and $\Omega_\Lambda$, using data from a suite of different
cosmological observations, and perform a comparison with the standard
$\Lambda$CDM model, containing both cold dark matter and a cosmological
constant. First considering a flat cosmology, we find that the UDM model with
affine EoS fits the joint observations very well, better than $\Lambda$CDM,
with best fit values $\alpha=0.01 \pm 0.02$ and $\Omega_\Lambda=0.70 \pm 0.04$
(95% confidence intervals). The standard model (best fit
$\Omega_\Lambda=0.71\pm 0.04$), having one less parameter, is preferred by a
Bayesian model comparison. However, the affine EoS is at least as good as the
standard model if a flat curvature is not assumed as a prior for $\Lambda$CDM.
For the latter, the best fit values are $\Omega_K=-0.02^{+0.01}_{-0.02} $ and
$\Omega_\Lambda=0.71 \pm 0.04$, i.e. a closed model is preferred. A phantom UDM
with affine EoS is ruled out well beyond $3\sigma$.
