Two evolutionary tracks for the merger of two carbon/oxygen white dwarfs of a combined mass exceeding the Chandrasekhar mass. In these scenarios the first phase of mass transfer is dynamically stable. On the left an example of the stable mass transfer channel is shown, on the right the formation reversal channel. In the latter scenario the first phase of mass transfer is dynamically stable which results in a low-mass helium-star with a long lifetime. The initially less massive star becomes the first formed white dwarf. The top and bottom parts of the figure have different scales due to a common envelope phase, denoted as CE in the figure. Abbreviations are as in Fig. 4. Additionally sRLOF stands for stable Roche lobe overflow, HG is a Hertzsprung-gap star and He G a helium giant.