Type changes of zero mean curvature surfaces of Riemann-type in the Lorentz-Minkowski 3-space

A zero mean curvature surface in the Lorentz-Minkowski 3-space is said to be of Riemann-type if it is foliated by circles and at most countably many straight lines in parallel planes. We classify all zero mean curvature surfaces of Riemann-type according to their causal characters, and we give new examples of ZMC surfaces containing lightlike lines and a zero mean curvature entire graph of mixed type.