086388

In Xu et al. openPar2018closePar we report the detection of very extended openParr_500>15 arcminclosePar X- ray galaxy groups not included in any previous X-ray or SZ cluster catalogs. We select 4 of these groups whose X-ray fluxes are above the nominal flux-limits of previous RASS cluster catalogs openPar>3 x10elev-12 erg selev-1 cmelev-2 in the 0.1-2.4 keV energy bandclosePar to be followed up by XMM-Newton. They have low mass openParvirgul10elev13 M_SunclosePar comma are at low redshift openParz<0.08closePar comma and appear to exhibit flatter surface brightness distributions than usual. The XMM-Newton observations will allow us to determine the reasons why this potentially new class of objects has been missed in previous X-ray surveys and the impact on the sample incompleteness when using galaxy cluster counts for cosmology.