R. de la Llave, S. Tompaidis Nature of singularities for analyticity domains of invariant curves (103K, Plain TeX) ABSTRACT. We present theoretical arguments (based on infinite dimensional bifurcation theory) and numerical evidence (based on non-perturbative methods) that the boundaries of analyticity of invariant curves can be described as an accumulation of branch points, which are typically of order 2. We show how this fact would explain previous numerical results of several authors and how it suggests more efficient numerical algorithms, which we implement.