WHOLE EARTH TELESCOPE OBSERVATIONS AND SEISMOLOGICAL ANALYSIS OF THE COOL ZZ-CETI STAR GD-154

This paper presents the results of high speed photometric observations of the cool variable DA white dwarf (DAV) GD 154 obtained with the Wh ole Earth Telescope. GD 154 is one of the coolest pulsating DA white d warfs and its study is important for understanding the red edge of the ZZ Ceti instability strip. Its power spectrum is dominated by three i ndependent modes (P-1 = 1186.5s, P-2 = 1088.6s and P-3 = 402.6s), and their harmonics and linear combinations. None of the half-integer harm onics reported in previous observations were present during the WET ca mpaign. We propose that the observed modes are trapped in the thin out er hydrogen layers. From the resulting identification of the pulsation modes, one derives an estimate of the rotation period (2.3 days) and of the mass of the outer hydrogen layer (2 x 10(-10)M).