  Within our program of physical characterization of trans-Neptunian objects
and centaurs, we predicted a stellar occultation by the centaur (54598) Bienor
to occur on January 11, 2019, with good observability potential. We obtained
high accuracy astrometric data to refine the prediction, resulting in a shadow
path favorable for the Iberian Peninsula. This encouraged us to carry out an
occultation observation campaign that resulted in five positive detections from
four observing sites. This is the fourth centaur for which a multichord (more
than two chords) stellar occultation has been observed so far, the other three
being (2060) Chiron, (10199) Chariklo, and (95626) 2002 GZ$_{32}$. From the
analysis of the occultation chords, combined with the rotational light curve
obtained shortly after the occultation, we determined that Bienor has an
area-equivalent diameter of $150\pm20$ km. This diameter is $\sim30$ km smaller
than the one obtained from thermal measurements. The position angle of the
short axis of the best fitting ellipse obtained through the analysis of the
stellar occultation does not match that of the spin axis derived from long-term
photometric models. We also detected a strong irregularity in one of the minima
of the rotational light curve that is present no matter the aspect angle at
which the observations were done. We present different scenarios to reconcile
the results from the different techniques. We did not detect secondary drops
related to potential rings or satellites. Nonetheless, similar rings in size to
that of Chariklo's cannot be discarded due to low data accuracy.
