In this paper we provide a systematic treatment of Willmore surfaces with orientation reversing symmetries and illustrate the theory by (old and new) examples. We apply our theory to isotropic Willmore two-spheres in $S^4$ and derive a necessary condition for such ( possibly branched) isotropic surfaces to descend to (possibly branched) maps from $\mathbb{ R} P^2$ to $S^4$. The Veronese sphere and several other examples of non-branched Willmore immersions from $\mathbb{ R} P^2$ to $S^4$ are derived as an illustration of the general theory. The Willmore immersions of $\mathbb{ R} P^2$, just mentioned and different from the Veronese sphere, are new to the authors' best knowledge.
Categories: math.DG