  Context: The observations of solar-like oscillations in evolved stars have
brought important constraints on their internal rotation rates. To correctly
reproduce these data, an efficient transport mechanism is needed in addition to
meridional circulation and shear instability. Aims: We study the efficiency of
the transport of angular momentum during the subgiant phase. Results: The
precise asteroseismic measurements of both core and surface rotation rates
available for the six Kepler targets enable a precise determination of the
efficiency of the transport of angular momentum needed for each of these
subgiants. These results are found to be insensitive to all the uncertainties
related to the modelling of rotational effects before the post-main sequence
phase. An interesting exception in this context is the case of young subgiants
(typical values of log(g) close to 4), because their rotational properties are
sensitive to the degree of radial differential rotation on the main sequence.
These young subgiants constitute therefore perfect targets to constrain the
transport of angular momentum on the main sequence from asteroseismic
observations of evolved stars. As for red giants, we find that the efficiency
of the additional transport process increases with the mass of the star during
the subgiant phase. However, the efficiency of this undetermined mechanism
decreases with evolution during the subgiant phase, contrary to what is found
for red giants. Consequently, a transport process with an efficiency that
increases with the degree of radial differential rotation cannot account for
the core rotation rates of subgiants, while it correctly reproduces the
rotation rates of red giant stars. This suggests that the physical nature of
the additional mechanism needed for the internal transport of angular momentum
may be different in subgiant and red giant stars.
