Stroemgren photometry of ZZ Ceti and other DA white dwarfs

Stroemgren colors for a sample of 71 stars classified as DA white dwarfs are presented. Comparison with the recent model atmospheres of the Kiel group indicates that the average gravity of 63 stars of the sample is log g = 7.98 +- 0.31, with no indication of a dependence on the effective temperature. This is consistent with the theoretical expectation that white dwarfs evolve at constant gravity. Correlations between Stroemgren colors and Greenstein multichannel colors are also given, using 52 stars that have been observed in common in the two systems. These correlations are used to compare the data with a second set of model atmospheres computed by Shipman. Contrary to what has been suggested in the past, no fundamental differences are found when confronting the Stroemgren data with the DA model atmospheres of Shipman or those of the Kiel group. Finally, observations of 11 pulsating objects reveal the existence of a narrow instability strip in the range 13 000 K> or approx. =T/sub e/> or approx. =11 000 K in a [(u-b), (b-y)] two-color diagram. The present results add evidence to the contention that all DA white dwarfs evolve to become ZZ Ceti pulsators in the instability strip