The 83 galaxies assigned to the RX J105453.3+552102 density peak in Fig. 4. Crosses indicate galaxies which were considered as interlopers on the basis of the gapper method. Circles indicate the 78 fiducial group members. The galaxy with the double symbol (i.e. SDSS J105454.00+552129.2) was not rejected to be more conservative. This choice did not affect the results of our dynamical analysis.