The Circles on the Diagonals of a Complete Quadrilateral as Diameters are Coaxal. Furthermore, the Orthocenters of the four Triangles of a Complete Quadrilateral are Collinear on the Radical Axis of the Coaxal Circles.
See also Coaxal Circles, Collinear, Complete Quadrilateral, Diagonal (Polygon), Orthocenter, Radical Axis
References
Johnson, R. A. Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle. Boston, MA:
Houghton Mifflin, p. 172, 1929.