We report on our findings of the bright , pulsating , helium atmosphere white dwarf GD 358 , based on time-resolved optical spectrophotometry .
We identify 5 real pulsation modes and at least 6 combination modes at frequencies consistent with those found in previous observations .
The measured Doppler shifts from our spectra show variations with amplitudes of up to 5.5 km s ^ { -1 } at the frequencies inferred from the flux variations .
We conclude that these are variations in the line-of-sight velocities associated with the pulsational motion .
We use the observed flux and velocity amplitudes and phases to test theoretical predictions within the convective driving framework , and compare these with similar observations of the hydrogen atmosphere white dwarf pulsators ( DAVs ) .
The wavelength dependence of the fractional pulsation amplitudes ( chromatic amplitudes ) allows us to conclude that all five real modes share the same spherical degree , most likely , \ell = 1 .
This is consistent with previous identifications based solely on photometry .
We find that a high signal-to-noise mean spectrum on its own is not enough to determine the atmospheric parameters and that there are small but significant discrepancies between the observations and model atmospheres .
The source of these remains to be identified .
While we infer T _ { \mathrm { eff } } \thickspace = 24 kK and \log g \sim 8.0 from the mean spectrum , the chromatic amplitudes , which are a measure of the derivative of the flux with respect to the temperature , unambiguously favour a higher effective temperature , 27 kK , which is more in line with independent determinations from ultra-violet spectra .