yeah, thats correct, he was. I forget the exact race, but until prost clinched the championship towards the back end of the 1993 season he indeed held both titles.

Jaques Villeneuve came closest to doing the same in 1996. Villeneuve is also one of three(the others are Mario Andretti and Emerson Fittipaldi) to win the Indycar championship, the indy 500 and the F1 world championship. Mansell came close to this in 1993 but lost the lead of the 500 during a late restart.

I guess Villeneuve could match Graham Hill this year as hes racing in  Le Mans - should he be in the winning car, then he will have won the 500, the F1 world championship, and Le Mans - also adding the Indycar championship to the list! Alas, like Mansell, he never won at monaco!