Properties of the period change after the first stable mass transfer phase. The period change distributions for binaries that produce DWDs in the current Galaxy are shown in the black histogram, and the corresponding cumulative distributions are shown in red solid lines. Left and right panels are for Ge’s and polytropic models, respectively. Here, P_1, mt refers to the orbital period after the termination of the first mass transfer phase and P_i is the initial orbital period of primordial binary. We note that most binaries with P_1, mt/P_i > 1 would enter into the CE processes in the subsequent evolution and some of the resulted DWDs have very short orbital periods (merge within the Hubble timescale). The DWD mergers lead to the decrease of the number for binaries with P_1, mt/P_i > 1. The results without considering the DWD mergers are shown in grey histograms and grey dashed lines. It is clear that the number of DWDs with very short orbital periods decreases a lot due to the DWD mergers. For binaries experiencing CE processes in the first mass transfer phase, the period changes of P_1, mt/P_i is less than 0.05, which are not shown for concision.