  At radio wavelengths, solar-type stars emit thermal free-free and
gyroresonance, gyrosynchrotron, and impulsive coherent emission. Thermal
free-free emission originates at layers where the optical depth is close to
unit, while high brightness temperature, variable emission, can be due to
flares via gyrosynchrotron emission. We observed the alpha Cen system with the
Australian Telescope Compact Array at 2 GHz for three days and 17 GHz for one
day. Both stars have been detected at 17 GHz, while only an upper limit has
been obtained at low frequency despite the longer integration time. The
brightness temperatures are consistent with the temperature of the upper
chromosphere of the Sun. Inverting the formulae of the free-free emission, the
average electron density of the plasma has been inferred. The same procedure
was applied to the data in the millimetre recently acquired with ALMA. A
comparison with the atmospheric solar models reveals a higher level of activity
in alpha Cen B rather than in alpha Cen A, even if still at quiescent level.
The non detection at 2 GHz allows us to put a lower limit in the filling factor
of active regions. The claimed detection of an Earth size planet in close orbit
to alpha Cen B, although doubtful, opens the opportunity to check the existence
of Star-Planet Magnetic Interaction (SPMI). We constructed dynamic spectra in
the 1.3 - 2.9 GHz of the 2 stars to search for time-variable coherent emission
but obtained a null result.
