  The observational study of stars in the sub-millimetre regime has only rather
recently begun and was made possible mainly by the Atacama Large
Millimeter/submillimeter Array (ALMA). The emission mechanisms of this
radiation from normal MS stars and its physical significance for the outer
atmospheric layers is the topic of intense contemporary study. Our previous
ALMA observations of the alpha Centauri binary system detected the submm
emission originating in the chromospheres of these solar-type stars.
Observations at another epoch are aiming at further characterising these
atmospheric layers and their behaviour with time. The comparison of data from
two epochs should present the basis for more advanced theoretical modelling. In
Cycle 4, both stars were again detected in the same bands as in the earlier
Cycle 2. These early data suggested a flattening of the SED towards longer
wavelengths. Now, the SEDs exhibit a single spectral slope over the entire
frequency range (90 to 675 GHz). The SEDs from Cycle 4 can be fit by power laws
of the form S(f)~f^a with a=1.76+/-0.01 for alpha Cen A and a=1.71+/-0.02 for
alpha Cen B. In addition, we were aiming at clarifying the status of the
recently discovered U source and its relation to the alpha Cen system. Proper
motion data of the U source should establish its relation to the alpha Cen
system, and the submm SED of U should provide information about its physical
nature. For the U source, the upper limits on its proper motion are much
smaller than the pm of alpha Cen, which essentially excludes any physical
relationship with the binary. For the the ALMA-SED of U applies a=2.55+/-0.14.
If this emission from U is due to dust, its opacity exponent b=a-2 would be
about 0.5, indicative of particle sizes that are larger than those of the
interstellar medium, where b(ISM)~2), but comparable to those found in
circumstellar discs.
