2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/68550We give an explicit construction and clasification of some very special sort of Enriques surfaces in characteristic two. This proves the existence of some of the surfaces that were called ``extra-special'' by Cossec and Dolgachev in their book on the subject, and whose existence was left open by them. These surfaces coincide with a class of surfaces that are introduced in recent work by Ekedahl and Shepherd--Barron.10 pagesAlgebraic Geometry14J28text