  Single crystals of the compounds LaFePO, PrFePO, and NdFePO have been
prepared by means of a flux growth technique and studied by electrical
resistivity, magnetic susceptibility and specific heat measurements. We have
found that PrFePO and NdFePO display superconductivity with values of the
superconducting critical temperature T_c of 3.2 K and 3.1 K, respectively. The
effect of annealing on the properties of LaFePO, PrFePO, and NdFePO is also
reported. The LnFePO (Ln = lanthanide) compounds are isostructural with the
LnFeAsO_{1-x}F_x compounds that become superconducting with T_c values as high
as 55 K for Ln = Sm. A systematic comparison of the occurrence of
superconductivity in the series LnFePO and LnFeAsO_{1-x}F_x points to a
possible difference in the origin of the superconductivity in these two series
of compounds.

Superconductivity