  Gas giants orbiting close to hot and massive early-type stars can reach
dayside temperatures that are comparable to those of the coldest stars. These
"ultra-hot Jupiters" have atmospheres made of ions and atomic species from
molecular dissociation and feature strong day-to-night temperature gradients.
Photometric observations at different orbital phases provide insights on the
planet atmospheric properties. We analyse the photometric observations of
WASP-189 acquired with the instrument CHEOPS to derive constraints on the
system architecture and the planetary atmosphere. We implement a light curve
model suited for asymmetric transit shape caused by the gravity-darkened
photosphere of the fast-rotating host star. We also model the reflective and
thermal components of the planetary flux, the effect of stellar oblateness and
light-travel time on transit-eclipse timings, the stellar activity and CHEOPS
systematics. From the asymmetric transit, we measure the size of the ultra-hot
Jupiter WASP-189 b, $R_p=1.600^{+0.017}_{-0.016}\,R_J$, with a precision of 1%,
and the true orbital obliquity of the planetary system $\Psi_p=89.6\pm1.2\deg$
(polar orbit). We detect no significant hotspot offset from the phase curve and
obtain an eclipse depth $\delta_\text{ecl}=96.5^{+4.5}_{-5.0}\,\text{ppm}$,
from which we derive an upper limit on the geometric albedo: $A_g<0.48$. We
also find that the eclipse depth can only be explained by thermal emission
alone in the case of extremely inefficient energy redistribution. Finally, we
attribute the photometric variability to the stellar rotation, either through
superficial inhomogeneities or resonance couplings between the convective core
and the radiative envelope.
