  ASCC 123 is a little-studied young and dispersed open cluster. Recently, we
conducted the first research devoted to it. In this paper, we complement our
previous work with TESS photometry for the 55 likely members of the cluster. We
pay special attention to seven of these high-probability members, all with FGK
spectral types, for which we have high-resolution spectra from our preceding
work. By studying the TESS light curves of the cluster members we determine the
rotational period and the amplitude of the rotational modulation for 29
objects. The analysis of the distribution of the periods allows us to estimate
a gyrochronogical age for ASCC 123 similar to that of the Pleiades, confirming
the value obtained in our previous investigation. A young cluster age is also
suggested by the distribution of variation amplitudes. In addition, for those
stars with spectroscopic data we calculate the inclination of their rotation
axis. These values appear to follow a random distribution, as already observed
in young clusters, with no indication of spin alignment. However, our sample is
too small to confirm this on more solid statistical grounds. Finally, for these
seven stars we study the level of magnetic activity from the H$\alpha$ and CaII
H&K lines. Despite the small number of data points, we find a correlation of
the H$\alpha$ and CaII flux with Rossby number. The position of these stars in
flux--flux diagrams follows the general trends observed in other active
late-type stars.
