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The optical identification of large number of X-ray sources such as thosefrom the ROSAT All-Sky Survey is challenging with conventional spectroscopicfollow-up observations. We investigate two ROSAT All-Sky Survey fields of size10 * 10 degrees each, one at galactic latitude b = 83 deg (Com), the other at b= -5 deg (Sge), in order to optically identify the majority of sources. We usedoptical variability, among other more standard methods, as a means ofidentifying a large number of ROSAT All-Sky Survey sources. All objects fainterthan about 12 mag and brighter than about 17 mag, in or near the error circleof the ROSAT positions, were tested for optical variability on hundreds ofarchival plates of the Sonneberg field patrol.  The present paper contains probable optical identifications of altogether 256of the 370 ROSAT sources analysed. In particular, we found 126 AGN (some ofthem may be misclassified CVs), 17 likely clusters of galaxies, 16 eruptivedouble stars (mostly CVs), 43 chromospherically active stars, 65 stars brighterthan about 13 mag, 7 UV Cet stars, 3 semiregular resp. slow irregular variablestars of late spectral type, 2 DA white dwarfs, 1 Am star, 1 supernova remnantand 1 planetary nebula.  X-ray emission is, expectedly, tightly correlated with optical variability,and thus our new method for optically identifying X-ray sources is demonstratedto be feasible.

Optical counterparts of ROSAT X-ray sources in two selected fields at low vs. high Galactic latitudes