Formation of ultracold $^{39}$K$^{133}$Cs Feshbach molecules

We report the creation of an ultracold gas of bosonic \potasBnospace \csisotope molecules. We first demonstrate a cooling strategy relying on sympathetic cooling of \csisotope to produce an ultracold mixture. From this mixture, weakly bound molecules are formed using a Feshbach resonance at 361.7 G. After atom cleanout the molecular gas contains $7.6(10)\times 10^3$ molecules with a lifetime about 100 ms, limited by two-body decay. We perform Feshbach spectroscopy to observe several new resonances and characterize the bound state used for magnetoassociation spectroscopically. Finally, we fit the combined results to obtain improved K-Cs interaction potentials. This provides a good starting point for the creation of ultracold samples of ground-state 39K133Cs molecules.