PHOTOMETRIC AND SPECTROSCOPIC ANALYSES OF COOL HDC STARS

A photometric and spectroscopic survey of many of the cool H-deficient Carbon (HdC) stars, including the R Coronae Borealis (RCB) stars, has been undertaken. For the RCB stars we have data on both the low amplitude photometric variations at maximum light and the major declines, the latter being the most distinguishing feature between the RCB and HdC stars. The photometric data at maximum light have been analysed using the Lomb-Scargle Fourier method, which has revealed many significant periodicities. From estimates of the temperature of these stars, an observational period-temperature diagram can be compared with theoretical models. These models have also enabled us to determine the evolutionary lifetime of these stars for comparison with observational parameters, e.g., period changes. Over the last several years we have photoelectrically covered the declines (and the recovery from some of these declines) of a number of RCB stars, from which we have been able to characterise two extreme types of decline behaviour. In addition, we have for the very first time spectroscopically recorded the initial decline phase of one (the prototype, R CrB) of these stars.