On the eclipsing binary ELHC 10 with occulting dark disc in the Large Magellanic Cloud

Abstract

We investigate the luminous star ELHC 10 located in the bar of the Large Magellanic Cloud, concluding that it is a SB1 long-period eclipsing binary where the main eclipse is produced by an opaque structure hiding the secondary star. For the more luminous component we determine an effective temperature of 6500 $\pm$ 250 $K$, log\,g = 1.0 $\pm$ 0.5 and luminosity 5970 L$_{\sun}$. From the radial velocities of their photospheric lines we calculate a mass function of 7.37 $\pm$ 0.55 M$_{\sun}$. Besides Balmer and forbidden N II emission, we find splitting of metallic lines, characterized by strong discrete absorption components (DACs), alternatively seen at the blue and red side of the photospheric spectrum. These observations hardly can be interpreted in terms of an structured atmosphere but might reflect mass streams in an interacting binary. The primary shows signatures of s-process nucleosynthesis and might be a low-mass post-AGB star with a rare evolutionary past if the binary is semi-detached. The peak separation and constancy of radial velocity in H$\alpha$ suggest that most of the Balmer emission comes from a circumbinary disc.