Domains of analytic existence in real Frechet spaces

If E is a real separable Frechet space, we prove that every non void domain Omega of E is open for a continuous semi-norm is a domain of analytic existence. In particular, every non void, open and convex subset Omega of E is a domain of analytic existence. Moreover, this result cannot be improved in the case of an arbitrary real separable Frechet space. In fact, in the space omega of real sequences, a non void domain Omega is a domain of analytic existence if and only if Omega is open for a continuous semi-norm.