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We report on our findings of the bright, pulsating, helium atmosphere whitedwarf GD 358, based on time-resolved optical spectrophotometry. We identify 5real pulsation modes and at least 6 combination modes at frequencies consistentwith those found in previous observations. The measured Doppler shifts from ourspectra show variations with amplitudes of up to 5.5 km/s at the frequenciesinferred from the flux variations. We conclude that these are variations in theline-of-sight velocities associated with the pulsational motion. We use theobserved flux and velocity amplitudes and phases to test theoreticalpredictions within the convective driving framework, and compare these withsimilar observations of the hydrogen atmosphere white dwarf pulsators (DAVs).The wavelength dependence of the fractional pulsation amplitudes (chromaticamplitudes) allows us to conclude that all five real modes share the samespherical degree, most likely, l=1. This is consistent with previousidentifications based solely on photometry. We find that a high signal-to-noisemean spectrum on its own is not enough to determine the atmospheric parametersand that there are small but significant discrepancies between the observationsand model atmospheres. The source of these remains to be identified. While weinfer T_eff=24kK and log g~8.0 from the mean spectrum, the chromaticamplitudes, which are a measure of the derivative of the flux with respect tothe temperature, unambiguously favour a higher effective temperature, 27kK,which is more in line with independent determinations from ultra-violetspectra.

A new look at the pulsating DB white dwarf GD 358: Line-of-sight velocity measurements and constraints on model atmospheres