  The properties of Bose-Einstein condensed gases can be strongly altered by
tuning the external magnetic field near a Feshbach resonance. Feshbach
resonances affect elastic collisions and lead to the observed modification of
the scattering length. However, as we report here, this is accompanied by a
strong increase in the rate of inelastic collisions. The observed three-body
loss rate in a sodium Bose-Einstein condensation increased when the scattering
length was tuned to both larger or smaller values than the off-resonant value.
This observation and the maximum measured increase of the loss rate by several
orders of magnitude are not accounted for by theoretical treatments. The strong
losses impose severe limitations for using Feshbach resonances to tune the
properties of Bose-Einstein condensates. A new Feshbach resonance in sodium at
1195 G was observed.
