tokens: that there is a background and foreground sources contamination. Fig. 9 shows the stellar position of the proper motion members and Fig. 10 the parallax histogram. It can be seen that they are distributed throughout the analysed region. Figure 8. Histogram of stellar proper motion probability in the left overdensity region. The dotted line corresponds to 1σ a members, the full line indicates the lowest value of the probability of the members. Figure 9. Stellar positions of the proper motions members of the left overdensity (black dots). The red squares represent the location of the known O members. Figure 10. Histogram of parallaxes of 1σ a proper motion members in the left overdensity region. Table 7. Left overdensity proper motion members up to G = 17.5 mag. The columns gives the position (α, δ), the Gaia DR2 identifier, the parallax π and its error σ π , the proper motion components and their errors μ α cosδ , , μ δ , and the G magnitude. Full table is available online. 5.3 Open cluster (bottom overdensity) We now focus on the bottom overdensity. We first eliminate the Cygnus OB2 proper motion members obtained in Section 4.2 . After applying the procedure of Section 3 , the parameters of the new group are obtained (see Table 5 ). The membership probability of the stars is calculated by equation ( 4 ). 25 proper motion members of the new stellar group are identified (see Table 8 ). The index E takes the value of 0.94, this indicates that the group and field stars are almost completely separated. Fig. 11 shows the stellar position of the proper motion members. It can be seen that this group is an open cluster, whose centre is (α, δ) = (306.99820°, 40.54240°).The radius of 9 arcmin is determined from the radial surface density profile of the members (Fig. 12 ). Figure 11. Stellar positions of the proper motions members of the open cluster (black dots). The red squares represent the location of the known O members. Figure 12. Stellar density profile of the open cluster (bottom overdensity) as a function of the distance from the cluster centre. Then, we calculate the mean parallax of the members π a and estimate the distance d π a . Both values are presented in Table 5 . The estimated distance d π a = 1457 ± 11 pc indicates that this group is also located in the foreground of the main Cygnus OB2 group. Besides, an inspection of the magnitudes in Table 8 reveals that this open cluster is formed by faint stars. Recently, Castro-Ginard et al. ( 2020 ) found 582 new open clusters distributed along the Galactic disc performing a blind search for open clusters using Gaia DR2 data. We noticed that one of these objects, UBC585, corresponds to this open cluster. Our mean parallax and mean proper motion are in good agreement with the values obtained for UBC585, while we find eight more members for UBC585. Table 8. 25 proper motion members of the open cluster (bottom overdensity). 6 CONCLUSIONS In the Cygnus OB2 region, we analyse a sample of stars from
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