COMMISSIONS 27 AND 42 OF THE IAU
             INFORMATION BULLETIN ON VARIABLE STARS
                         Number 3978

                                                   Konkoly Observatory
                                                   Budapest
                                                   19 January 1994

                                                   HU ISSN 0374 - 0676


           THE Am STAR HD 43478 IS AN ECLIPSING BINARY


The star HD 43478 = BD +32d1246 was classified A3-F2-F5 from the CaII K line, the
H Balmer lines and the metallic lines respectively by Osawa (1965). It had been earlier
classified A3p SiSr by Bertaud (1959) and was, for that reason, included in a CORAVEL
survey of radial velocities of cool Ap stars. CORAVEL measurements made with the 1m
Swiss telescope at Observatoire de Haute-Provence (France) in 1986 showed this star is a
double-line spectroscopic binary. The orbital period is

               P = 5.46414 +- 0.00013 days,

the excentricity is negligible and the masses are

        M_1 * sin^3*i =1.76 +- 0.02, M_2 * sin^3*i =1.59 +- 0.02M_Sun.

This star had been observed in the Geneva photometric system at the same telescope and
was suspected of variability, so it was reobserved at the 76cm telescope of the Jungfraujoch
Observatory (Switzerland), especially in 1987, 1988 and 1989. Although bad weather
prevented a complete and dense phase coverage to be obtained, two well-defined minima
could be observed at both phases 0.25 and 0.75, which coincide with the epochs of zero
radial velocity (relatively to the systemic velocity). The lightcurve is shown in Figure 1.
HD 43478 is therefore an eclipsing binary, although the minima are no deeper than 0.1
magnitudes.
  The (U-B) and (B-V) indices do not vary in a significant way, showing that both 
components have nearly the same effective temperature. Since the components seem nearly
identical, it is not irrelevant to estimate their physical parameters from the colour 
indices of the system. HD 43478 has been measured in the uvbybeta system by two different 
groups and the average indices, found in the database of Hauck et al. (1990), give
E(b - y) = 0.029, T_eff = 6970 K and log g = 3.71, using the calibration by Moon &
Dworetsky (1985) with the correction on log g for Am stars (Dworetsky & Moon 1986).
Taking E(B2 - V1) =1.15 E(b - y) = 0.033 and using the calibration of Kobi & North
(1990), the Geneva indices give T_eff = 6850 K, log g = 3.72 and M = 1.82 M_Sun. The 
results from both photometric systems are in perfect agreement, the effective temperature
is exactly that expected for the hydrogen spectral type (F2) and the mass estimated from
the Geneva colours matches the spectroscopic masses extremely well. The components
of this system appear to be somewhat evolved on the main sequence. Combining the
photometric mass and surface gravity leads to a radius R = 3.1 R_Sun.
  The projected rotational velocities, estimated from the widths of the autocorrelation
peaks measured with CORAVEL, are 29.2 +- 0.9 and 20.5 +- 0.7 kms^-1 for the primary
and the secondary component respectively. If the axial rotation of both components is
synchronized with the orbital period, then the radius of the primary is exactly that inferred
from photometry, while the secondary has R = 2.2 R_Sun, implying log g = 4.0. From the
depths of the minima, a rough estimate gives i ~ 80d, so the masses of the primary and
of the secondary may be about 1.84 and 1.66 M_Sun respectively.

                                [FIGURE 1]

 Figure 1:  V magnitude of HD 43478 as a function of phase, according to the ephemeris given on 
 top of the figure. The zero phase corresponds here to the quadrature, not to an eclipse.

The discovery of this SB2 system tends to confirm the high rate of SB2 binaries among
Am stars, noticed by Abt & Levy (1985).


P. NORTH                                  B. NICOLET
Institut d'astronomie
                                          Observatoire de Geneve
de l'Universite de Lausanne               ch. des Maillettes 51
CH-1290 Chavannes-des-Bois                CH-1290 Sauverny
Switzerland                               Switzerland


References:
Abt, H.A., Levy, S.G., 1985, ApJS, 59, 229 [BIBCODE 1985ApJS...59..229A ]
Bertaud, C., 1959, J.Obs., 42, 45 [BIBCODE 1959JO.....42...45B ]
Dworetsky, M.M., Moon, T.T., 1986, MNRAS, 220, 787 [BIBCODE 1986MNRAS.220..787D ]
Hauck, B., Nitschelm, C., Mermilliod, M., 1990, A&AS, 85, 989 [BIBCODE 1990A&AS...85..989H ]
Kobi, D., North, P., 1990, A&AS, 85, 999 [BIBCODE 1990A&AS...85..999K ]
Moon, T.T., Dworetsky, M.M., 1985, MNRAS, 217, 305 [BIBCODE 1985MNRAS.217..305M ]
Osawa, K., 1965, Ann. Tokyo Astron.Obs., 2nd series, IX, no. 3