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

We report the creation of an ultracold gas of bosonic ${}^{39}$K${}^{133}$Cs molecules. We first demonstrate a cooling strategy relying on sympathetic cooling of ${}^{133}$Cs to produce an ultracold mixture. From this mixture, weakly bound molecules are formed using a Feshbach resonance at 361.7 G. The molecular gas contains $7.6(10)\times {10}^3$ molecules with a lifetime of about 130 ms, limited by two-body decay. We perform Feshbach spectroscopy to observe several new interspecies resonances and characterize the bound state used for magnetoassociation. 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 ${}^{39}$K${}^{133}$Cs molecules.