  A fast transition between a standard matter-like era and a late $\Lambda$
cold dark matter ($\Lambda$CDM)-like epoch (or more in general, a CDM+DE era),
generated by a single Unified Dark Matter component, can provide a new
interesting paradigm in the context of general relativity, alternative to
$\Lambda$CDM itself or other forms of DE or modified gravity theories invoked
to explain the observed acceleration of the Universe. UDM models with a fast
transition have interesting features, leading to measurable predictions, thus
they should be clearly distinguishable from $\Lambda$CDM (and alternatives)
through observations. Here we look at different ways of prescribing
phenomenological UDM models with fast transition, then focusing on a
particularly simple model. We analyse the viability of this model by studying
features of the background model and properties of the adiabatic UDM
perturbations, which depend on the effective speed of sound and the functional
form of the Jeans scale. As a result, theoretical constraints on the parameters
of the models are found that allow for a behaviour compatible with
observations.
