“…The boundary regularity theory for fractional Laplacian, developed in [31,32,37,38], ensures that, for a solution u to (1.1) with Ω of class C 2 , the quantity (∂ ν ) s u is well defined. Natural Hopf's Lemmas were then proved in [24,Proposition 3.3] and [30,Lemma 1.2], and constituted the base point in the study of overdetermined problems for the fractional Laplacian, see [17,24,30,34,44]. In this paper we consider overdetermined problems of the type…”