Christopher J. Bradley, David Monk, and Geoff C. Smith,
On a porism associated with the Euler and Droz-Farny lines,
Forum Geometricorum, 7 (2007) 11--17.
Abstract: The envelope of the Droz-Farny lines of a triangle is
determined to be the inconic with foci at the circumcenter and orthocenter
by using purely Euclidean means. The poristic triangles sharing this inconic
and circumcircle have a common circumcenter, centroid and orthocenter.