Cs and Rb fountains

We discuss the present performance and limits of our Cs and Rb fountains. BNM-SYRTE operates three cold atom microwave fountains: two Cs fountains and a dual Cs-Rb fountain. By using an ultra-stable cryogenic sapphire oscillator to probe the atoms, the frequency stability reaches 3.5 /spl times/ 10/sup -14//spl tau//sup -1/2/ which will allow an improvement of the accuracy below the present 10/sup -15/. We discuss here the problems to be solved for reaching a 10/sup -16/ accuracy. We also use the fountains to test the stability of the fine structure constant. Measurements of the ratio v/sub Rb//v/sub Cs/ spread over a two year interval show no change of a at the 7 /spl times/ 10/sup -15//year level.