  UV spectroscopy and spectropolarimetry hold the key to understanding certain
aspects of massive stars that are largely inaccessible with optical or longer
wavelength observations. This is especially true for the rapidly-rotating Be
and Bn stars, owing to their high temperatures, geometric asymmetries, binary
properties, and evolutionary history. UV spectropolarimetric observations are
extremely sensitive to the photospheric consequences of rapid rotation (i.e.
oblateness, temperature, and surface gravity gradients). Our polarized
radiative-transfer modelling predicts that with low-resolution UV
spectropolarimetry covering 120 -- 300 nm the inclination angle of a rapid
rotator can be determined to within 5 degrees, and the rotation rate to within
1%. The origin of rapid rotation in Be/n stars can be explained by either
single-star or binary evolution, but their relative importance is largely
unknown. Some Be stars have hot sub-luminous (sdO) companions, which at an
earlier phase transferred their envelope (and with it mass and angular
momentum) to the present-day rapid rotator. Through spectral modelling of a
wide range of simulated Be/n+sdO configurations, we demonstrate that
high-resolution high-SNR UV spectroscopy can detect an sdO star even when
$\sim$1,000 times fainter in the UV than its Be/n star companion. This degree
of sensitivity is needed to more fully explore the parameter space of Be/n+sdO
binaries, which so far has been limited to about a dozen systems with
relatively luminous sdO stars. We suggest that a UV spectropolarimetric survey
of Be/n stars is the next step forward in understanding this population. Such a
dataset would, when combined with population synthesis models, allow for the
determination of the relative importance of the possible evolutionary pathways
traversed by these stars, which is also crucial for understanding their future
evolution and fate.
