Comparison of observed PSPC spectral distributions with the ISM model (cold absorber) predictions. The open symbols and dashed line represent the observations and best fit to the 1991 April 21 data. The filled symbols and solid line represent the observations and best fit to the 1993 April 29 data. One sigma errors bars are shown. This illustrates that the spectral distributions of three of the four stars have remained essentially constant. The exception is Cyg OB2 No. 8A where a substantial change occurred shortward of 2 keV. Although the Cyg OB2 No. 9 best‐fit models suggest that a change occurred, confirmation is not possible owing to the noise level of the observations.