THE VARIABILITY OF A NEWLY DISCOVERED GAMMA-DORADUS STAR, HD-108100

A coordinated photometric campaign at five observatories to monitor th e suspected brightness variations of the early-l; star HD 108100 was u ndertaken by the DSN (Delta Scuti Network) and WET (Whole Earth Telesc ope). 288 hours of high-quality observations during a 58 day time span led to the discovery of two frequencies of 1.32 and 1.40 c/d with V a mplitudes of 0.010 and 0.007 mag. The detection of both frequencies is statistically significant with amplitude signal/noise ratios of 18 an d 12. The light variations are accompanied by v - y color variations o f 0.005 and 0.003 mag, respectively. The variability of HD 108100 is t ypical of the new class of gamma Dor variables. We also derive amplitu de ratios and phase differences between the light curves at different wavelengths. These are consistent with g-mode pulsation of l = 1, but not l = 2. The observed periods can be fit by two modes with successiv e radial orders of n similar to 19, Studies of the delta Scuti star 4 CVn from 1970 to 1976 reported the existence of unusually low frequenc ies of 1.3 and 1.4 c/d. These frequencies can now be traced back to HD 108100, the single comparison star used in these studies. Consequentl y, 4 CVn should no longer be regarded as an example of a star exhibiti ng both delta Scuti type pulsation (acoustic modes of mixed character) and gamma Dor variability (gravity modes of high order). An analysis of unpublished data from 1970 to 1976 shows that the amplitudes associ ated with the two frequencies of HD 108100 are variable and that the t wo frequencies can appear to switch their power.