  Azimuthal distributions of pi+, K+ and K- mesons have been measured in Au+Au
reactions at 1.5 AGeV and Ni+Ni reactions at 1.93 AGeV. In semi-central
collisions at midrapidity, pi+ and K+ mesons are emitted preferentially
perpendicular to the reaction plane in both collision systems. In contrast for
K- mesons in Ni+Ni reactions an in-plane elliptic flow was observed for the
first time at these incident energies.
