  We report the observation of interspecies Feshbach resonances in an optically
trapped mixture of $^{85}$Rb and $^{133}$Cs. We measure 14 interspecies
features in the lowest spin channels for a magnetic field range from 0 to 700 G
and show that they are in good agreement with coupled-channel calculations. The
interspecies background scattering length is close to zero over a large range
of magnetic fields, permitting the sensitive detection of Feshbach resonances
through interspecies thermalisation. Our results confirm the quality of the
Rb-Cs potential curves and offer promising starting points for the production
of ultracold polar molecules.

Atomic Physics