  To characterise the long-term stability and true photometric sensitivity of a
space-based instrument is a difficult task and can be done best with
independent measurements of a relatively quiet star. A rare occasion for such a
test are the complementary observations of some bright Plejades stars with the
Kepler/K2 telescope and the BRITE-Austria (BAb) and UniBRITE (UBr) satellites.
While most of them show a complex oscillatory behaviour, the frequency spectrum
of the bright B-type star Atlas is relatively simple. From the 71 days-long K2
observations we extract the three dominant frequencies and show that the
residuals have a noise level of less than 10ppm. While the BAb observations are
not sensitive enough, we find the same periodicities in the 167 days-long UBr
data set, which shows no additional significant signal down to a noise level of
about 100ppm. This impressively demonstrates the stability of the BRITE
instruments in the low-frequency regime and their capability to detect sub-mmag
variability, even for stars close to the faint end of the nominal dynamic
range.
