New high time- and spectral-resolution observations of,' Cas have revealed an intermittent periodicity ( days) in emission line profile variations. Profiles of a number of weak spectral lines have been obtained including two He I double emission lines and some 0-type absorption lines. Absorption- and emission-line profiles have been calculated for rotating stars by two independent methods. The observed profiles in the spectrum of,' Cas are fitted best by those computed for an 08 star rotating at break-up velocity and viewed at i = 550 + 100. A detailed model of the star and extended envelope is derived and explanations of the line profile and other spectral variations are proposed. The method is applicable to all rotating B stars.