  LaFe1-yCoyAsO1-xFx (x=0.11) with various y values were prepared and their
electrical resistivities, superconducting diamagnetisms and Hall coefficients
have been measured. 75As- and 135La-NMR studies have also been carried out In
spite of the successful Co-doping, we have not found any meaningful correlation
of Tc with y, which indicates that the Tc-suppression by Co-doping is
considered not to be so significant as expected for superconductors with nodes.
Even for superconductors without nodes, it may not be easy to expect this small
effect on Tc, if there are two different (disconnected) Fermi surfaces whose
order parameters have opposite signs. The data of the NMR Knight shift indicate
that the Cooper pairs are in the singlet state and the spin susceptibility is
almost fully suppressed at low temperature.
