  \ersn\, is considered, together with \erti, as a realization of the XY
antiferromagnet on the pyrochlore lattice. We present magnetization
measurements confirming that \ersn\, does not order down to 100 mK but exhibits
a freezing below 200 mK. Our neutron scattering experiments evidence the strong
XY character of the \er moment and point out the existence of short range
correlations in which the magnetic moments are in peculiar configurations, the
Palmer-Chalker states, predicted theoretically for an XY pyrochlore
antiferromagnet with dipolar interactions. Our estimation of the \ersn\,
parameters confirm the role of the latter interactions on top of relatively
weak and isotropic exchange couplings.
