Contribution to the search of binaries among Am stars II. HD 81976 and HD 98880, double-lined spectroscopic binaries

We present the results of the observations of two Am stars of eighth magnit ude, the double-lined spectroscopic binaries HD 81976 and HD 98880, carried out with the CORAVEL instrument at the Observatoire de Haute-Provence in o rder to determine their orbital elements, We found 1) for HD 81976: P = 5.0 55750 days. T = 2449785.941 HJD. omega = 341.4 degrees, e = 0.061, K-1 = 61 .68 km s(-1), K-2 = 63.84 km s(-1), V-0 = 19.85 km s(-1) a(1); sin i = 4.78 8 Gm. a(2) sin i = 4.956 Gm, M-1 sin(3) i = 0.5875 M circle dot, M-2 sin(3) i = 0.5675 M circle dot, and 2) for HD 98880; P = 14.20783 days, T-0 = 244 8682.883 HJD (ascending node), e = 0, K-1 = 42.47 km s(-1), K-2 = 49.16 km s(-1), V-0 = 2.40 km s(-1), a(1) sin i = 8.238 Gm, M-1 sin i = 9.604 Gm, M- 1 sin(3) i = 0.6091 M circle dot, M-2 sin(3) i = 0.5262 M circle dot. The f irst of these two systems, HD 81976, is formed by two quasi-identical stars , and the Hipparcos data (M-V, B-V) are consistent with late A stars in eff ective temperature: it is likely that the components rotate synchronised wi th the orbital motion. A third body may be present in this system since (i) the orbit has a significant eccentricity despite its short period and (ii) the systemic velocity V-0 shows a possible drift, For the second system, H D 98880, me give Deltam(B) = 1.25 and we propose a simple model based upon Stromgren photometric indices and the HR theoretical diagram of Schaller et al (1992) in addition to orbital parameters and Hipparcos data: T-eff = 70 00 K, log(10) g = 4.0. M-1 = 1.9 M circle dot, M-2 = 1.6 M circle dot, log( 10) (age) = 9.12. The components do not rotate synchronously contrary to HD 81976. Both binaries appear to be detached systems without possibility of eclipses.