NONVARIABILITY AMONG LAMBDA-BOOTIS STARS

With asteroseismic techniques it is possible to investigate the interi or and the evolutionary status of stars via their frequency spectrum. Both information would be very much needed for lambda Bootis stars, a group of metal-poor Population I, A-type stars, since no conclusive th eory exists explaining the observed abundance anomalies. Geneva and St romgren photometry place these stars inside the classical instability strip or at least very close to it. We therefore have started an exten sive photometric survey for pulsation in lambda Bootis stars and have discovered so far 13 new variables. In this paper we present results f or stars which presumably are constant, because we are able to establi sh only an upper level for possible variability. A typical noise level of 3 mmag for Stromgren b was achieved in the relevant frequency doma in up to 100 d(-1). Considering the given noise level of our survey, w e conclude that at least 50% of all investigated lambda Bootis stars i nside the instability strip are pulsating, making this group remarkabl e compared to stars with similar spectral types. This may suggest that a low (surface) metallicity has an influence on the pulsation behavio r of stars. New models, as well as further observations are needed to clarify the lambda Bootis phenomenon using asteroseismic techniques.