The puzzling binary HD 143418

Abstract

Context. HD 143418 was discovered recently to be a double-lined spectroscopic binary with a primary designated as a CP star. Its light displays an orbital phase coupled variability with a peak-to-peak amplitude up to 0.04 mag. Aims. The photometry available and new high dispersion spectra were investigated from a point of view of CP characteristics. Methods. A series of high resolution high S/N coudé spectra was acquired from which 25 weak to strong unblended lines of Fe I and II, Ti II, Cr II, Zr II, and Ba II were selected to study spectral line variability. Two Zeeman spectra were obtained to search for a possible magnetic field of the star, and one echelle spectrum in a wide spectral region was analysed for abundance determination by means of synthetic spectra. The photometric observations were subjected to a PCA disentangling of the complex photometric behaviour. Results. We identified spectral lines of the secondary in the yellow region on the echelle as well as on two coudé spectra, whose occurence belongs to an F6V star and the intensity corresponds to the luminosity ratio 0.06. Equivalent widths of the selected spectral lines of the primary component do not change within the errors of measurements. The spectra taken with a Zeeman analyser do not indicate a magnetic field. The abundance pattern does not correspond to characteristics of a variable CP2 star. The only remarkable deviation is a more than 1.1 dex deficit of scandium, one of the properties of non-variable Am stars. The photometric variability is tied to the orbital period and is due to ellipticity of the primary component and not to a putatively structured surface of the primary that is confirmed to rotate subsynchronously. The seasonal component of the light curve changes in amplitude as well as in shape. Conclusions. We conclude that the primary is a normal, mildly evolved A5V main sequence star. The seasonal variability of the orbitally modulated light curves may be related to an expected incidence of circumstellar matter originating in the tidally spinning up primary component. HD 143418 may be a prototype of a rare detached interacting close binary containing a subsynchronously rotating primary passing through its synchronisation stage.