%0 Journal Article %T A note on some overdetermined elliptic problem %A Fr¨¦d¨¦ric H¨¦lein %A Laurent Hauswirth %A Frank Pacard %J Mathematics %D 2010 %I arXiv %X We define the notion of an exceptional manifold to be a flat Riemannian manifold with boundary which supports a positive harmonic function satisfying simultaneously a zero Dirichlet condition and a constant (nonzero) Neumann condtion at the boundary. We study the two-dimensional case: we present various examples and give a general construction algorithm of such surface by using complex analysis. We deduce a classification of all such surfaces assuming some further natural hypotheses and prove a Bernstein type theorem. %U http://arxiv.org/abs/1001.1060v1