A LONG-TERM X-RAY VARIABILITY STUDY OF THE O-TYPE STARS SIGMA-ORIONISAND ZETA-ORIONIS

X-ray emission in early-type OB stars is thought to be generated by sh ock-heated gas in the radiatively driven wind of these stars. Calculat ions of the X-ray production for such a scenario depend on the underly ing shock structures, especially the occurrence rate of shocks, coolin g length and cooling time which directly influence the source location of the X-rays in the stellar wind. We present a detailed variability analysis of the available ROSAT data for our two program stars sigma O ri and zeta Ori. The long-term analysis of both stars covers a time ra nge of 3 years and includes seven pointed PSPC observations, an additi onal pointed HRI observation for sigma Ori and the ROSAT all-sky surve y data of both stars. In the case of sigma Ori we find no evidence for variability on all analysed time scales. Over a long time range the t iming analysis of the X-ray light curve of zeta Ori provides also no e vidence for variability. Only during a period of 2 days (September 23- 25 1992) did we detect a moderate increase in X-ray count rate by appr oximate to 15%.