  The Centaur (10199) Chariklo has the first rings system discovered around a
small object. It was first observed using stellar occultation in 2013. Stellar
occultations allow the determination of sizes and shapes with kilometre
accuracy and obtain characteristics of the occulting object and its vicinity.
Using stellar occultations observed between 2017 and 2020, we aim at
constraining Chariklo's and its rings physical parameters. We also determine
the rings' structure, and obtain precise astrometrical positions of Chariklo.
We predicted and organised several observational campaigns of stellar
occultations by Chariklo. Occultation light curves were measured from the data
sets, from which ingress and egress times, and rings' width and opacity were
obtained. These measurements, combined with results from previous works, allow
us to obtain significant constraints on Chariklo's shape and rings' structure.
We characterise Chariklo's ring system (C1R and C2R), and obtain radii and pole
orientations that are consistent with, but more accurate than, results from
previous occultations. We confirmed the detection of W-shaped structures within
C1R and an evident variation of radial width. The observed width ranges between
4.8 and 9.1 km with a mean value of 6.5 km. One dual observation (visible and
red) does not reveal any differences in the C1R opacity profiles, indicating
ring particle's size larger than a few microns. The C1R ring eccentricity is
found to be smaller than 0.022 (3-sigma), and its width variations may indicate
an eccentricity higher than 0.005. We fit a tri-axial shape to Chariklo's
detections over eleven occultations and determine that Chariklo is consistent
with an ellipsoid with semi-axes of 143.8, 135.2 and 99.1 km. Ultimately, we
provided seven astrometric positions at a milliarcseconds accuracy level, based
on Gaia EDR3, and use it to improve Chariklo's ephemeris.
