OME 1994 Problem 2

Let $Oxyz$ be a trihedron whose edges $x,y, z$ are mutually perpendicular. Let $C$ be the point on the ray $z$ with $OC = c$ . Points $P$ and $Q$ vary on the rays $x$ and $y$ respectively in such a way that $OP+OQ = k$ is constant. For every $P$ and $Q$ , the circumcenter of the sphere through $O,C,P,Q$ is denoted by $W$ . Find the locus of the projection of $W$ on the plane O $xy$ . Also find the locus of points $W$ .