SIMBAD references

Empirical location of the ZZ Ceti instability strip, as well as atmospheric parameters of individual objects, is extremely difficult to determine, and the literature values of T_eff often differ by 2000 K. For optical spectra, very small systematic errors of observations or models have a large influence on the result. In the UV the situation is slightly better, but the spectra with some certainty can provide only a relation between T_eff and log g. All literature values, that rely on spectroscopy only, are unreliable; accurate determinations require the use of additional constraints like parallaxes, V magnitudes or gravitational redshifts.