HIGH-SPEED PHOTOMETRIC-OBSERVATIONS OF THE PULSATING DA WHITE-DWARF GD-165

New high-speed photometric observations of the pulsating DA white dwarf GD 165 are presented. The Fourier spectrum of the light curve of GD 165 exhibits two main regions of power at 120 and 193 s. The presence of a high-amplitude long period mode near approximately 1800 s reported by Bergeron and McGraw is not confirmed by these new observations. Light curves obtained with the Canada-France-Hawaii Telescope reveal previously undetected low-amplitude harmonic oscillations. Observations with the Whole Earth Telescope are used to resolve the two principal regions of power. The 120 and 193 s peaks are shown to be multiplets composed of at least three, and possibly five frequency components. The most likely explanation is that these two peaks correspond to nonradial gravity modes with different values of the radial order k and with l = 1 or 2 split into 2l + 1 components by slow rotation. The frequency differences observed between the three dominant components of each peak are consistent with equal spacing, but a slight asymmetry in spacing cannot be totally ruled out at this stage. They suggest a rotation time scale of the order of 4.2 days. Within a peak, power is not distributed evenly or symmetrically among the three frequency components, particularly for the 193 s mode. Consequently, a simple interpretation in terms of geometric effects (inclination of the pulsation axis with respect to the line of sight) cannot be invoked here. The tentative identification of modes with l = 1 or 2 is used to constrain the hydrogen layer mass in GD 165. In particular, the 120 s pulsation mode has a period sufficiently short that useful limits can be derived on this quantity. Using recent adiabatic pulsation calculations and new determinations of the atmospheric parameters of GD 165, it is found that M(H)/M* greater than or similar to 10(-3.7) or greater than or similar to 10(-6.4) if the 120 s pulsation is a mode with l = 1 or 2, respectively.