@article{JGG_2008_12_2_JGG_2008_12_2_a4,
     author = {H.-P. Schr�cker },
     title = {Double {Tangent} {Circles} and {Focal} {Properties} of {Sphero-Conics}},
     journal = {Journal for geometry and graphics},
     pages = {161--169},
     publisher = {mathdoc},
     volume = {12},
     number = {2},
     year = {2008},
     url = {http://geodesic.mathdoc.fr/item/JGG_2008_12_2_JGG_2008_12_2_a4/}
}