  Thanks to large dedicated surveys, large-scale magnetic fields have been
detected for about 10% of early-type stars. We aim to precisely characterize
the large-scale magnetic field of the magnetic component of the wide binary $o$
Lup, by using high-resolution ESPaDOnS and HARPSpol spectropolarimetry to
analyse the variability of the measured longitudinal magnetic field. In
addition, we investigate the periodic variability using space-based photometry
collected with the BRITE-Constellation by means of iterative prewhitening. The
rotational variability of the longitudinal magnetic field indicates a rotation
period $P_{\mathrm{rot}}=2.95333(2)$d and that the large-scale magnetic field
is dipolar, but with a significant quadrupolar contribution. Strong differences
in the strength of the measured magnetic field occur for various chemical
elements as well as rotational modulation for Fe and Si absorption lines,
suggesting a inhomogeneous surface distribution of chemical elements. Estimates
of the geometry of the large-scale magnetic field indicate $i=27\pm
10^{\circ}$, $\beta = 74^{+7}_{-9}\,^{\circ}$, and a polar field strength of at
least 5.25 kG. The BRITE photometry reveals the rotation frequency and several
of its harmonics, as well as two gravity mode pulsation frequencies. The
high-amplitude g-mode pulsation at $f=1.1057\mathrm{d^{-1}}$ dominates the
line-profile variability of the majority of the spectroscopic absorption lines.
We do not find direct observational evidence of the secondary in the
spectroscopy. Therefore, we attribute the pulsations and the large-scale
magnetic field to the B5IV primary of the $o$ Lup system, but we discuss the
implications should the secondary contribute to or cause the observed
variability.
