Orbital periods P and chirp mass ℳ of DWDs at birth. The DWD formation time is represented by the colour scale, while their merger time by GW emission is represented by dashed-dotted contours (both timescales are expressed in Gyr). The figure shows only a fraction of the population with orbital periods accessible to LISA. Dashed lines represent approximate boundaries between DWDs of different types: CO+CO, CO+He and He+He; ONe white dwarfs represent a negligible fraction of the population and occupy the same part of the parameter space as CO+He ones. We also note that DWDs in this figure do not encode any information about the type of the host galaxy.