A&A Supplement series, Vol. 127, January II 1998, 277-294
Received October 30, 1996; accepted May 9, 1997
,M. Künzli - P. North
Send offprint request: P. North
Institut d'Astronomie de l'Université de Lausanne,
CH-1290 Chavannes-des-Bois, Switzerland
We test the hypothesis of Berthet (1992) which foresees that Am
stars become giant metallic A and F stars (defined by an enhanced value of
the blanketing parameter 
of the Geneva photometry) when they
evolve.
If this hypothesis is right, Am and metallic A-FIII stars need to have the
same rate of binaries and a similar distribution of 
. From our new
spectroscopic data and from and radial velocities in the literature,
we show that it is not the case. The metallic giant stars are often fast
rotators with larger than 100 kms-1, while the maximum rotational
velocity for Am stars is about 100 kms-1. The rate of tight binaries with
periods less than 1000 days is less than 30% among metallic giants, which is
incompatible with the value of 75% for Am stars (Abt & Levy
1985). Therefore, the simplest way to explain the existence of giant
metallic F stars is to suggest that all normal A and early F stars might go
through a short "metallic" phase when they are finishing their life on the main sequence.
Besides, it is shown that only giant stars with spectral type comprised
between F0 and F6 may have a really enhanced value, while all A-type
giants seem to be normal.
keywords: stars: binaries: spectroscopic -- stars: chemically
peculiar -- stars: evolution -- stars: rotation -- stars: 
Scuti
line