Context : 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 Main-Sequence stars and its physical significance for the outer atmospheric layers is the topic of intense contemporary study .

Aims : Our previous ALMA observations of the \alpha { Cen } tauri 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 .
In addition , we were aiming at clarifying the status of the recently discovered U source and its relation to the \alpha { Cen } system .

Methods : The comparison of data from two epochs should present the basis for more advanced theoretical modelling of the chromospheres of \alpha { Cen } A and B .
Proper motion data of the U source should establish its relation to the \alpha { Cen } system , and U ’ s submm spectral energy distribution ( SED ) should provide information about its physical nature .

Results : 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 .
By analogy with the Sun , this was not expected .
Eventually , it turned out to be caused by an obsolete calibration , but this has now been remedied .
Each SED exhibits now a single spectral slope over the entire frequency range ( 90 to 675 GHz ) .
For the U source , the upper limits on its proper motion ( pm ) are much smaller than the pm of \alpha { Cen } , which essentially excludes any physical relationship with the binary .

Conclusions : The second epoch ALMA observations of \alpha { Cen } did not confirm the flattening of the SED in the lowest frequency bands that was reported before .
Rather , this was the result of an inadequate flux calibration using the minor planet Ceres .
Over the entire frequency range observed with ALMA , the SEDs from Cycle 4 can be fit by power laws of the form S _ { \nu } \propto \nu ^ { \alpha } with \alpha = 1.76 \pm 0.01 for \alpha { Cen } A and \alpha = 1.71 \pm 0.02 for \alpha { Cen } B .
For the infrared/submm background object U applies \alpha = 2.55 \pm 0.14 .
If this emission from U is due to dust , its opacity exponent \beta = \alpha - 2 would be about 0.5 , indicative of particle sizes that are larger than those of the interstellar medium ( \beta _ { ISM } \sim 2 ) , but comparable to those found in circumstellar discs .