Nodal domains are studied both for real $psi_R$ and imaginary part $psi_I$ of the wavefunctions of an open microwave cavity and found to show the same behavior as wavefunctions in closed billiards. In addition we investigate the variation of the number of nodal domains and the signed area correlation by changing the global phase $phi_g$ according to $psi_R+ipsi_I=e^{iphi_g}(psi_R+ipsi_I)$. This variation can be qualitatively, and the correlation quantitatively explained in terms of the phase rigidity characterising the openness of the billiard.
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