  We present time series photometry of Bienor in four observation campaigns
from 2013 to 2016 and compare them with previous observations in the literature
dating back to 2000. The results show a remarkable decline in the amplitude of
the rotational light curve and in the absolute magnitude. This suggests that
the angle between the rotation axis and the line of sight has changed
noticeably during the last 16 years as Bienor orbits the Sun. From the light
curve amplitude data we are able to determine the orientation of the rotation
axis of Bienor ($\beta_p=50\pm3^{\circ}$, $\lambda_p=35\pm8^{\circ}$). We are
also able to constrain the $b/a$ axial ratio of a triaxial Jacobi ellipsoidal
body (with semi-axis $a>b>c$). The best fit is for $b/a=0.45\pm0.05$, which
corresponds to a density value of $594^{+47}_{-35}$ kg$\,$m$^{-3}$ under the
usual assumption of hydrostatic equilibrium and given that Bienor's rotational
period is 9.17 h. However, the absolute magnitude of Bienor at several epochs
is not well reproduced. We tested several explanations such as relaxing the
hydrostatic equilibrium constraint, a large North-South asymmetry in the
surface albedo of Bienor or even a ring system. When a ring system of similar
characteristics to those of Chariklo and Chiron is included, we can fit both
the light curve amplitude and absolute magnitude. In this case the derived
axial ratio is modified to $b/a=0.37\pm0.10$. The implied density is
$678^{+209}_{-100}$ kg$\,$m$^{-3}$. Also the existence of a ring is consistent
with the spectroscopic detection of water ice on Bienor. Nevertheless the other
explanations cannot be discarded.
