The spin density matrix element \rzz of prompt \Dstar mesons as a function of \pt in the rapidity interval $0.3 |y| 0.8$ compared to that of inclusive \Jpsi mesons in the rapidity interval $2.5< y>4$ measured in Pb--Pb collisions at $\snn=5.02 \TeV$ in the 30--50\% centrality class. The bottom panel shows the deviation of the measurements from the null hypothesis ($\rzz = 1/3$) in units of the total uncertainties $\sigma_{\rzz}$. < /y>