  Azimuthal distributions of $\eta$ and $\pi^{0}$ mesons emitted at midrapidity
in collisions of 1.9 AGeV $^{58}$Ni+$^{58}$Ni and 2 AGeV $^{40}$Ca+$^{nat}$Ca
are studied as a function of the number of projectile-like spectator nucleons.
The observed anisotropy corresponds to a negative elliptic flow signal for
$\eta$ mesons, indicating a preferred emission perpendicular to the reaction
plane. The effect is smallest in peripheral Ni+Ni collisions. In contrast, for
$\pi^{0}$ mesons, elliptic flow is observed only in peripheral Ni+Ni
collisions, changing from positive to negative sign with increasing pion
transverse momentum.
