Observation of heteronuclear feshbach molecules from a 85Rb-87Rb gas

We report on the observation of ultracold heteronuclear Feshbach molecules. Starting with a Rb-87 Bose-Einstein condensate and a cold atomic gas of Rb-85, we utilize previously unobserved interspecies Feshbach resonances to create up to 25 000 molecules. Even though the Rb-85 gas is nondegenerate, we observe a large molecular conversion efficiency due to the presence of a quantum degenerate Rb-87 gas; this represents a key feature of our system. We compare the molecule creation at two different Feshbach resonances with different magnetic-field widths. The two Feshbach resonances are located at 265.44 +/- 0.15 G and 372.4 +/- 1.3 G. We also directly measure the small binding energy of the molecules through resonant magnetic-field association.