The President Pro Tempore of the United Federation of Planets is an office in the government of the Federation which is normally vacant, but which becomes occupied by a Federation Councillor appointed by the full Council upon the death, resignation, or disability of the sitting President. Under the Articles of the Federation, the President Pro Tempore is empowered to discharge the duties and responsibilities of the presidency for a temporary period. During this time, the Council is legally obligated to call for a special election and to determine who may qualify for presidential candidacy. When there is a President Pro Tempore, there is no sitting President. Upon the election of a new President, the President Pro Tempore is removed from that office, which again becomes vacant, and retains his or her position on the Council. (TNG - A Time to... novel: A Time for War, A Time for Peace)
Federation Councillor Ra'ch B'ullhy of Damiano served in this position for one standard month after the resignation of Min Zife in 2379. (TNG - A Time to... novel: A Time for War, A Time for Peace)
The term of a President Pro Tempore had been increased by statute to sixty standard days by 2385. That year Federation Councillor Baras Rodirya, under the identity of Ishan Anjar of Bajor, was appointed to serve as President Pro Tempore following the assassination of President Nanietta Bacco. (ST - The Fall novels: Revelation and Dust, A Ceremony of Losses, The Poisoned Chalice, Peaceable Kingdoms)