An investment casting core combination for use in casting an airfoil, said core combination comprising: a first ceramic core molded over a refractory metal core, the refractory metal core having: a leading edge and an intact leading portion extending aft from said leading edge a trailing edge and an intact trailing portion extending forward from said trailing edge, and a plurality of legs (46;52) separated by respective apertures (44;50); characterised in that: said leading edge is concave; said trailing edge is convex; said refractory metal core comprises: an intermediate portion over which said first ceramic core is molded; a first spanwise array of legs extending from said first ceramic core, respective first ends of said first spanwise array of legs merging with said intermediate portion and respective second ends of said first spanwise array of legs merging with said intact leading portion; a second spanwise array of legs extending from said first ceramic core, respective first ends of said second spanwise array of legs merging with said intermediate portion and respective second ends of said second spanwise array of legs merging with said intact trailing portion; said legs (46; 52) for forming cooling passages in the airfoil; and said first ceramic core fills an array of apertures in said intermediate portion of the refractory metal core. a first ceramic core molded over a refractory metal core, the refractory metal core having: a leading edge and an intact leading portion extending aft from said leading edge a trailing edge and an intact trailing portion extending forward from said trailing edge, and a plurality of legs (46;52) separated by respective apertures (44;50); characterised in that: said leading edge is concave; said trailing edge is convex; said refractory metal core comprises: an intermediate portion over which said first ceramic core is molded; a first spanwise array of legs extending from said first ceramic core, respective first ends of said first spanwise array of legs merging with said intermediate portion and respective second ends of said first spanwise array of legs merging with said intact leading portion; a second spanwise array of legs extending from said first ceramic core, respective first ends of said second spanwise array of legs merging with said intermediate portion and respective second ends of said second spanwise array of legs merging with said intact trailing portion; said legs (46; 52) for forming cooling passages in the airfoil; and said first ceramic core fills an array of apertures in said intermediate portion of the refractory metal core. a leading edge and an intact leading portion extending aft from said leading edge a trailing edge and an intact trailing portion extending forward from said trailing edge, and a plurality of legs (46;52) separated by respective apertures (44;50); characterised in that: said leading edge is concave; said trailing edge is convex; said refractory metal core comprises: an intermediate portion over which said first ceramic core is molded; a first spanwise array of legs extending from said first ceramic core, respective first ends of said first spanwise array of legs merging with said intermediate portion and respective second ends of said first spanwise array of legs merging with said intact leading portion; a second spanwise array of legs extending from said first ceramic core, respective first ends of said second spanwise array of legs merging with said intermediate portion and respective second ends of said second spanwise array of legs merging with said intact trailing portion; said legs (46; 52) for forming cooling passages in the airfoil; and said first ceramic core fills an array of apertures in said intermediate portion of the refractory metal core. said leading edge is concave; said trailing edge is convex; said refractory metal core comprises: an intermediate portion over which said first ceramic core is molded; a first spanwise array of legs extending from said first ceramic core, respective first ends of said first spanwise array of legs merging with said intermediate portion and respective second ends of said first spanwise array of legs merging with said intact leading portion; a second spanwise array of legs extending from said first ceramic core, respective first ends of said second spanwise array of legs merging with said intermediate portion and respective second ends of said second spanwise array of legs merging with said intact trailing portion; said legs (46; 52) for forming cooling passages in the airfoil; and said first ceramic core fills an array of apertures in said intermediate portion of the refractory metal core. an intermediate portion over which said first ceramic core is molded; a first spanwise array of legs extending from said first ceramic core, respective first ends of said first spanwise array of legs merging with said intermediate portion and respective second ends of said first spanwise array of legs merging with said intact leading portion; a second spanwise array of legs extending from said first ceramic core, respective first ends of said second spanwise array of legs merging with said intermediate portion and respective second ends of said second spanwise array of legs merging with said intact trailing portion; said legs (46; 52) for forming cooling passages in the airfoil; and said first ceramic core fills an array of apertures in said intermediate portion of the refractory metal core.