  In their LAMA'2016 paper Gau, Wang and Wu conjectured that a partial isometry
$A$ acting on $\mathbb{C}^n$ cannot have a circular numerical range with a
non-zero center, and proved this conjecture for $n\leq 4$. We prove it for
operators with $\mathrm{rank}\,A=n-1$ and any $n$. The proof is based on the
unitary similarity of $A$ to a compressed shift operator $S_B$ generated by a
finite Blaschke product $B$. We then use the description of the numerical range
of $S_B$ as intersection of Poncelet polygons, a special representation of
Blaschke products related to boundary interpolation, and an explicit formula
for the barycenter of the vertices of Poncelet polygons involving elliptic
functions.
