Collisional properties of sympathetically cooled 39 K

We report experimental evidence of the sympathetic cooling of {sup 39}K with {sup 87}Rb down to 1 {mu}K, obtained in a tight-confining magnetic trap. This allowed us to perform a direct measurement of the elastic cross section of {sup 39}K below 50 {mu}K. The result obtained for the triplet scattering length, a{sub T}=-51(7) Bohr radii, agrees with previous results derived from photo association spectra and from Feshbach spectroscopy of {sup 40}K.