Formation of ultracold <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts> <mml:mi mathvariant="normal">K</mml:mi> <mml:mprescripts/> <mml:none/> <mml:mn>39</mml:mn> </mml:mmultiscripts> <mml:mmultiscripts> <mml:m

We report the creation of an ultracold gas of bosonic <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mmultiscripts> <a:mi mathvariant="normal">K</a:mi> <a:mprescripts/> <a:none/> <a:mn>39</a:mn> </a:mmultiscripts> <a:mmultiscripts> <a:mi>Cs</a:mi> <a:mprescripts/> <a:none/> <a:mn>133</a:mn> </a:mmultiscripts> </a:mrow> </a:math> molecules. We first demonstrate a cooling strategy relying on sympathetic cooling of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mmultiscripts> <c:mi>Cs</c:mi> <c:mprescripts/> <c:none/> <c:mn>133</c:mn> </c:mmultiscripts> </c:math> 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 <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mn>7.6</d:mn> <d:mrow> <d:mo>(</d:mo> <d:mn>10</d:mn> <d:mo>)</d:mo> </d:mrow> <d:mo>×</d:mo> <d:msup> <d:mn>10</d:mn> <d:mn>3</d:mn> </d:msup> </d:mrow> </d:math> 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 <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mmultiscripts> <e:mi mathvariant="normal">K</e:mi> <e:mprescripts/> <e:none/> <e:mn>39</e:mn> </e:mmultiscripts> <e:mmultiscripts> <e:mi>Cs</e:mi> <e:mprescripts/> <e:none/> <e:mn>133</e:mn> </e:mmultiscripts> </e:mrow> </e:math> molecules.