The C(K)onstance Hypertext System (KHS) is an elaborated example of an open hypertext System. Open hypertext Systems combine heterogeneous resources and different hypertext applications such as e-mail, text and image hypertexts with access to online databases or other information resources. These hypertexts, to which heterogeneous users have access, grow steadily in size. This paper takes an object-oriented approach and proposes a rigorous typing of hypertext objects to overcome the problems of open hypertext. The advantage of this typing is shown on an e-mail hypertext as a special application of the KHS system. KHS is written in Smalltalk and is being developed on UNIX-machines.