  We test the viability of a single fluid cosmological model containing a
transition from a dark-matter-like regime to a dark-energy-like regime. The
fluid is a k-essence scalar field with a well-defined Lagrangian. We constrain
its model parameters with a combination of geometric probes and conclude that
the evidence for this model is similar to the evidence for $\Lambda$CDM. In
addition, we find a lower bound for the rapidity of the transition, implying
that fast transitions are favored with respect to slow ones even at background
level.
