  We previously reported a rare class of variable star light curves isolated
from a sample of 4.7 million candidate variables from the ATLAS survey. Dubbed
`UCBH' light curves, they have broad minima and narrow, symmetrical maxima,
with typical periods of 1-10 days and amplitudes of 0.05--0.20 mag. They
maintain constant amplitude, shape, and phase coherence over multiple years,
but do not match any known class of pulsating variables. A localized bright
spot near the equator of a rotating star will produce a UCBH-type light curve
for most viewing geometries. Most stars that exhibit rotational variability
caused primarily by a single bright feature should therefore appear as UCBH
stars, although a rotating bright spot is not the only thing that could produce
a UCBH-type lightcurve. We have spectroscopically investigated fourteen UCBH
stars and found ten of them to be Ap/Bp stars: A-type or B-type stars with
greatly enhanced photospheric abundances of specific heavy elements.
Rotationally variable Ap/Bp stars are referred to as $\alpha^2$ CVn variables.
Most ATLAS UCBH stars are therefore $\alpha^2$ CVn stars, although only a
minority of $\alpha^2$ CVn stars in the literature have UCBH light curves. The
fact that $\alpha^2$ CVn stars dominate the UCBH class suggests that lone
bright spots with sufficient size and contrast develop more readily on Ap/Bp
stars than on any other type. The $\alpha^2$ CVn UCBH stars may be
characterized by a specific magnetic field topology, making them intriguing
targets for future Zeeman-Doppler imaging.
