Benin is governed by the constitution of 2 December 1990. This constitution is resolutely presidential. However, from 9 April 1996 to 14 May 1998 there was a prime minister, Adrien Houngbédji. The constitution was not changed to make provision for a prime minister. It was simply the result of a presidential decree. As far as I can tell, Prime Minister Houngbédji was responsible to the legislature during this time.
The story, I think, was that Mathieu Kérékou was elected as president at the March 1996 election. He defeated the incumbent, President Nicéphore Soglo. Kérékou was slightly behind Soglo at the first ballot, but at the second, where only two candidates can stand, he won a plurality. Moreover, he did so with the help of Adrien Houngbédji, from the PRD party, who came third at the first round of the election. In return, President Kérékou was obliged to reward Houngbédji and the post of prime minister was created. In 1998 Prime Minister Houngbédji stepped down and the post of prime minister was not renewed.
Was Benin semi-presidential from 1996-98 (and, again, I am assuming that the PM was responsible to the legislature during this time)? The answer is ‘no’, because the constitution was still clearly presidential and, in any case, there was no collective responsibility. However, it is interesting that Benin had what might be called a dual-authority system for a period. For some people, a behavioural definition of this sort is still what is understood as semi-presidentialism. All the same, I am willing to bet that those same people do not include Benin in their studies of dual-authority systems.
So, I do not include Benin in my list of historic semi-presidential countries, but the 1996-1998 period is an interesting case nonetheless.
(This is an updated version of a previous post).