Dupin cyclide

A torus seen through an inversion: the tube swells on one side and thins on the other, yet every line of curvature is still a circle. As the parameter d moves the tube pinches and crosses itself.

x = (d(c − a cos u cos v) + b² cos u) / (a − c cos u cos v)

On mathcurve.com as Cyclide de Dupin