  We present a model atmosphere analysis of optical VRI and infrared JHK
photometric data of about two dozen ZZ Ceti stars. We first show from a
theoretical point of view that the resulting energy distributions are not
particularly sensitive to surface gravity or to the assumed convective
efficiency, a result which suggests a parameter-free effective temperature
indicator for ZZ Ceti stars. We then fit the observed energy distributions with
our grid of model atmospheres and compare the photometric effective
temperatures with the spectroscopic values obtained from fits to the hydrogen
line profiles. Our results are finally discussed in the context of the
determination of the empirical boundaries of the ZZ Ceti instability strip.
