SIMBAD references

We are obtaining high signal-to-noise (S/N) optical spectra and time-series photometry of all known DA4 and DA5 white dwarfs brighter than B = 16.0 mag. Deriving their atmospheric parameters, masses and limits of variability, we can study the statistics of the ZZ Ceti instability strip. We have found 14 non-variables in agreement with mass stratification of the instability strip, but also at least one non-variable star inside the instability strip that cannot be explained by total mass alone.