X-ray source populations in the region of the open clusters NGC 6633 and IC 4756

Using deep exposures (similar to 10(5) s) with the ROSAT High Resolution Im ager, we have performed flux-limited surveys for X-ray sources in the vicin ity of the Hyades-age open stellar clusters NGC 6633 and IC 4756, detecting 31 and 13 sources, respectively. Our primary aim is to search for so-far u nrecognized cluster members. We propose identifications or classifications (cluster member, field star, extragalactic field object) for the X-ray sour ces, based on published membership lists, and on X-ray:optical flux ratios and optical colour-magnitude diagrams. Results of simulating the expected X -ray-emitting source populations an compared with the ROSAT measurements an d with the expected capabilities of the XMM mission. The simulations provid e a novel method of comparing the activity levels of NGC 6633 and IC 4756 w ith that of the Hyades. The measurements and simulations confirm that clust er members are the major class of X-ray emitter in these fields at flux lev els f(X) greater than or equal to 10(14) erg cm(-2) s(-1) (0 1-2.4 keV), co ntributing similar to 40 per cent of the total X-ray sources. We find six p ossible new members in NGC 6633 and four candidates in IC 4756; all require further observation to establish membership probability.