  In this note we revisit the Kovacs effect, concerning the way in which the
volume of a glass-forming liquid, which has been driven out of equilibrium,
changes with time while the system evolves towards a metastable state. The
theoret- ical explanation of this phenomenon has attracted much interest even
in recent years, because of its relation with some subtle aspects of the still
elusive nature of the glass transition. In fact, even if there is a rather
general consensus on the fact that what is experimentally observed on cooling
is the dramatic effect produced by the dynam- ical arrest of slower degrees of
freedom over the experimental time scale, it is not yet clear whether this
phenomenology can be justified upon assuming the existence of an underlying
(possibly, high order) phase transition at lower temperatures.
