ON THE NEW LAMBDA-BOOTIS-TYPE SPECTROSCOPIC BINARY-SYSTEMS HD-84948 AND HD-171948

We report the discovery of two new lambda Bootis-type spectroscopic bi nary systems (HD 84948 and HD 171948). High resolution spectra were an alyzed in order to derive abundances as well as astrophysical quantiti es. It resulted in extreme (HD 171948) and moderate (HD 84948) underab undances of Na, Mg, Si, Ca, Sc, Ti, Cr and Fe with respect to the Sun, thus establishing all four individual stars as true members of the la mbda Bootis group. These two systems are therefore the first spectrosc opic binary systems where both components are lambda Bootis-type stars . Furthermore, we have discovered delta Scuti type pulsation for one c omponent of HD 84948 making this star particularly interesting for fur ther observations. Since many authors have used the rotational velocit y for discriminating lambda Bootis stars (upsilon sin i > 50 kms(-1)), we note that both components of HD 171948 are the slowest (upsilon si n i < 20 kms(-1)) rotating lambda Bootis-type stars known so far. Usin g the Hipparcos data, we are able to rule out the diffusion/mass-loss mechanism as origin for these two systems. Pre- as well as Main Sequen ce evolutionary tracks for the age determination yield consistent resu lts with the predictions of the diffusion/accretion theory, establishi ng both systems as being very close to the Main Sequence. A determinat ion of the orbital elements and/or an asteroseismological investigatio n of HD 84948 could provide an independent proof of our results.