  Differential production cross sections of K- and K+ mesons have been measured
in Ni+Ni and Au+Au collisions at a beam energy of 1.5 AGeV. The K-/K+ ratio is
found to be nearly constant as a function of the collision centrality and
system size. The spectral slopes and the polar emission pattern differ for K-
and K+ mesons. These observations indicate that K+ mesons decouple earlier from
the fireball than K- mesons.
