  The lambda Bootis stars are Population I, late B to early F-type stars, with
moderate to extreme (up to a factor 100) surface underabundances of most
Fe-peak elements and solar abundances of lighter elements (C, N, O, and S). To
put constraints on the various existing theories that try to explain these
peculiar stars, we investigate the observational properties of lambda Bootis
stars compared to a reference sample of normal stars. Using various photometric
systems and Hipparcos data, we analyze the validity of standard photometric
calibrations, elemental abundances, and Galactic space motions. There
crystallizes a clear picture of a homogeneous group of Population I objects
found at all stages of their main-sequence evolution, with a peak at about 1
Gyr. No correlation of astrophysical parameters such as the projected
rotational velocities or elemental abundances with age is found, suggesting
that the a-priori unknown mechanism, which creates lambda Bootis stars, works
continuously for late B to early F-type stars in all stages of main-sequence
evolution. Surprisingly, the sodium abundances seem to indicate an interaction
between the stars and their local environment.
