ROSAT X-RAY LIGHT CURVES OF EARLY-TYPE STARS - A SEARCH FOR X-RAY TIME VARIABILITY OF OB STARS

We have investigated ROSAT X-ray data of OB stars to search for eviden ce of time variability in the X-ray emission from early-type stars. As an example for such studies we present a detailed variability analysi s for our two program stars sigma Ori and zeta Ori which have been mul tiply observed with ROSAT. The long-term analysis of both stars now co vers a time range of 2.5 years and includes six pointed PSPC observati ons, an additional pointed HRI observation of sigma Ori and the ROSAT all-sky survey data of both stars. Over a long time range the X-ray li ght curves of sigma Ori and zeta Ori show no evidence for variability. In the case of zeta Ori we detected a moderate increase in X-ray coun t rate during a period of 2 days which can be explained as a strong sh ock propagating in the wind of an O-type star.