  To shed light on the symmetry of the superconducting order parameter in
Na_xCoO_2-yH_2O, the Mn doping effects are studied. X-ray absorption
spectroscopy verifies that the doped Mn impurities occupy the Co sites and are
with a valance close to +4. Impurity scattering by Mn is in the unitary limit
that, however, does not lead to strong Tc suppression. This absence of the
strong impurity effects on T_c is not consistent with the simple picture of a
sign-changing order parameter. Coexistence of the s-wave and unconventional
order parameters is proposed to reconcile all existing experiments and has been
directly observed by the specific heat experiments.
