X-rays from the open cluster NGC 6633

NGC 6633 is a young, open cluster with a similar age to the Hyades and Prae sepe, but probably a lower metallicity. We present the results of ROSAT Hig h Resolution Imager observations of an optically selected catalogue of like ly members of NGC 6633. 8 out of 51 NGC 6633 members have been detected, wi th main-sequence spectral types A to G, above a threshold X-ray luminosity of approximate to 6-12 x 10(28) erg s(-1). We find that NGC 6633 does not c ontain cool stars that are as X-ray luminous as the most active objects in the Hyades and that the median X-ray luminosity of F-G stars in NGC 6633 is less than that in the Hyades, but probably greater than in Praesepe. Howev er, when X-ray activity is expressed as the X-ray to bolometric flux ratio we find that NGC 6633 and the Hyades are very similar and display similar p eak levels of coronal activity. We attribute this discrepancy to a number o f possible wide binary systems with higher X-ray (and bolometric) luminosit ies in the Hyades sample and either a low metallicity in NGC 6633, which ma kes its cool stars both X-ray and bolometrically less luminous at the same colour, or a distance to NGC 6633 that has been underestimated, which would decrease stellar X-ray luminosities without changing X-ray to bolometric f lux ratios.