X-ray variability of the bright BL Lac object PKS 2155-304

We have observed the BL Lac object PKS 2155-304 with the EXOSAT observatory and with IUE on 1983 October 31 and November 29. At the same time, optical photometry has been obtained. Variability has been observed both on a time scale of hours during the October 31 observation and on the time scale of one month. During the November 29 observation the source was 2 times brighter than in the previous observation, but no spectral modification is found between the two observations. The source has been also detected in the ME instrument and an energy spectrum has been fitted with a power law. A marginal detection in the ME Xenon counters suggests a flattening of the spectrum in the high energy band. These X-ray data extending over three decades in energy are compared with the simultaneous UV and optical observations.