336 United Nations — Treaty Series • Nations Unies — Recueil des Traités_____1980 (c) The representative of the State has signed the treaty subject to ratification; or (d) The intention of the State to sign the treaty subject to ratification appears from the full powers of its representative or was expressed during the negotiation. 2. The consent of a State to be bound by a treaty is expressed by acceptance or approval under conditions similar to those which apply to ratification. Article 15. CONSENT TO BE BOUND BY A TREATY EXPRESSED BY ACCESSION The consent of a State to be bound by a treaty is expressed by accession when: (a) The treaty provides that such consent may be expressed by that State by means of accession; (b) It is otherwise established that the negotiating States were agreed that such con sent may be expressed by that State by means of accession; or (c) All the parties have subsequently agreed that such consent may be expressed by that State by means of accession. Article 16. EXCHANGE OR DEPOSIT OF INSTRUMENTS OF RATIFICATION, ACCEPTANCE, APPROVAL OR ACCESSION Unless the treaty otherwise provides, instruments of ratification, acceptance, approval or accession establish the consent of a State to be bound by a treaty upon: (a) Their exchange between the contracting States; (b) Their deposit with the depositary; or (c) Their notification to the contracting States or to the depositary, if so agreed. Article 17. CONSENT TO BE BOUND BY PART OF A TREATY AND CHOICE OF DIFFERING PROVISIONS 1. Without prejudice to articles 19 to 23, the consent of a State to be bound by part of a treaty is effective only if the treaty so permits or the other contracting States so agree. 2. The consent of a State to be bound by a treaty which permits a choice be tween differing provisions is effective only if it is made clear to which of the provi sions the consent relates. Article 18. OBLIGATION NOT TO DEFEAT THE OBJECT AND PURPOSE OF A TREATY PRIOR TO ITS ENTRY INTO FORCE A State is obliged to refrain from acts which would defeat the object and pur pose of a treaty when: (a) It has signed the treaty or has exchanged instruments constituting the treaty subject to ratification, acceptance or approval, until it shall have made its in tention clear not to become a party to the treaty; or (b) It has expressed its consent to be bound by the treaty, pending the entry into force of the treaty and provided that such entry into force is not unduly delayed. SECTION 2. RESERVATIONS Article 19. FORMULATION OF RESERVATIONS A State may, when signing, ratifying, accepting, approving or acceding to a treaty, formulate a reservation unless: (a) The reservation is prohibited by the treaty; Vol. 1155,1-18232