The variability of a newly discovered gamma Doradus star, HD 108100

A coordinated photometric campaign at ve obser- vatories to monitor the suspected brightness variations of the early-F star HD 108100 was undertaken by the DSN (Delta Scuti Network) and WET (Whole Earth Telescope). 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 amplitudes of 0.010 and 0.007 mag. The detection of both fre- quencies is statistically signicant with amplitude signal/noise ratios of 18 and 12. The light variations are accompanied by v y color variations of 0.005 and 0.003 mag, respectively. The variability of HD 108100 is typical of the new class of Dor variables. We also derive amplitude ratios and phase differences between the light curves at different wavelengths. These are consistent with g-mode pulsation of ' = 1, but not ' = 2.Theobservedperiodscanbetbytwomodeswithsuccessive radial orders of n 19. Studiesofthe Scutistar4CVnfrom1970to1976reported the existence of unusually low frequencies 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. Consequently, 4 CVn should no longer be regarded as an example of a star ex- hibiting both Scuti type pulsation (acoustic modes of mixed character) and Dor variability (gravity modes of high order). An analysis of unpublished data from 1970 to 1976 shows that the amplitudes associated with the two frequencies of HD 108100 are variable and that the two frequencies can appear to switch their power.