“…These surfaces, the so-called rotational ZMC surfaces, are classified: see for example [16,18,23]. More recently, there is a great activity in the search of explicit examples of ZMC surfaces of mixed type where the transition between the spacelike and timelike regions occurs along degenerate curves: here we refer [2,9,10,11,12,13,15,22] without to be a complete list.…”