  We study the isochronicity of centers at $O\in \mathbb{R}^2$ for systems
$\dot x=-y+A(x,y), \dot y=x+B(x,y)$, where $A, B\in \mathbb{R}[x,y]$, which can
be reduced to the Lienard type equation. Using the so-called C-algorithm we
have found 27 new multiparameter isochronous centers.
