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Up to now, the largest sample of galaxy clusters selected in X-rays comesfrom the ROSAT All-Sky Survey (RASS). Although there have been many interestingclusters discovered with the RASS data, the broad point spread function (PSF)of the ROSAT satellite limits the amount of spatial information of the detectedobjects. This leads to the discovery of new cluster features when are-observation is performed with higher resolution X-ray satellites. Here wepresent the results from XMM-Newton observations of three clusters:RXCJ2306.6-1319, ZwCl1665 and RXCJ0034.6-0208, for which the observationsreveal a double or triple system of extended components. These clusters belongto the extremely expanded HIghest X-ray FLUx Galaxy Cluster Sample(eeHIFLUGCS), which is a flux-limited cluster sample ($f_\textrm{X,500}\geq5\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$ in the $0.1-2.4$ keV energy band). Foreach structure in each cluster, we determine the redshift with the X-rayspectrum and find that the components are not part of the same cluster. This isconfirmed by an optical spectroscopic analysis of the galaxy members.Therefore, the total number of clusters is actually 7 and not 3. We deriveglobal cluster properties of each extended component. We compare the measuredproperties to lower-redshift group samples, and find a good agreement. Our fluxmeasurements reveal that only one component of the ZwCl1665 cluster has a fluxabove the eeHIFLUGCS limit, while the other clusters will no longer be part ofthe sample. These examples demonstrate that cluster-cluster projections canbias X-ray cluster catalogues and that with high-resolution X-ray follow-upthis bias can be corrected.

Projection effects in galaxy cluster samples: insights from X-ray redshifts