Binarity of Am stars in Praesepe and Hyades

CORAVEL radial-velocity observations of Am stars in the Hyades and Praesepe have allowed the determination of orbital elements for 10 spectroscopic bi naries, among which 3 are first determinations. One Am star (KW 40) is foun d to be a well hierarchised triple system. KW 538 has a rather long period (435 days) for an Am star. Orbits of systems with periods shorter than 8.5 days are circularized, or present eccentricities smaller than 0.04. For 19 Am stars, the number of quadruple-, triple-, double-, single systems is 1:2 :14(10+4?):(2?). The Am stars in a (beta, B - V) diagram clearly stand away by 0.03 mag from the sequence defined by normal main-sequence stars. This diagram could be a powerful method to identify Am stars in more distant open clusters, provi ded there is no differential reddening. In the colour-magnitude diagram (M- V, beta), double-lined binaries are 0.6 - 0.7 mag above the ZAMS as expecte d, while most single-lined are close to or on the ZAMS because the secondar y does not contribute much light. The absence of X-ray detection of 4 syste ms in the Hyades is an argument for the presence of a white dwarf secondary .