Tunable dual-species Bose-Einstein condensates of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">K</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>39</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">Rb</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>87</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>

We present the production of dual-species Bose-Einstein condensates (BECs) of $^{39}\mathrm{K}$ and $^{87}\mathrm{Rb}$. Preparation of both species in the $\left|F=1,{m}_{F}=\ensuremath{-}1\right\ensuremath{\rangle}$ state enabled us to exploit a total of three Feshbach resonances, which allows for simultaneous Feshbach tuning of the $^{39}\mathrm{K}$ intraspecies and the $^{39}\mathrm{K}\text{\ensuremath{-}}^{87}\mathrm{Rb}$ interspecies scattering length. Thus dual-species Bose-Einstein condensates were produced by sympathetic cooling of $^{39}\mathrm{K}$ with $^{87}\mathrm{Rb}$. A dark spontaneous force optical trap was used for $^{87}\mathrm{Rb}$ to reduce the losses in $^{39}\mathrm{K}$ due to light-assisted collisions in the optical trapping phase, which can be of benefit for other dual-species experiments. The tunability of the scattering length was used to perform precision spectroscopy of the interspecies Feshbach resonance located at 117.56(2) G and to determine the width of the resonance to 1.21(5) G by rethermalization measurements. The transition region from miscible to immiscible dual-species condensates was investigated and the interspecies background scattering length was determined to $28.5{a}_{0}$ using an empirical model. This paves the way for dual-species experiments with $^{39}\mathrm{K}$ and $^{87}\mathrm{Rb}$ BECs ranging from molecular physics to precision metrology.

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