Deconstructing HD 28867

The 3″ pair of B9 stars HD 28867 is one of the brightest X-ray sources in the Taurus-Auriga star-forming region. In this multiwavelength study, we attempt to deduce the source of the X-ray emission. We show that the east component is the X-ray source. The east component has a near-IR excess and displays narrow absorption lines in the optical, both of which are consistent with a cool stellar companion. This companion is one of the brightest low-mass pre–main-sequence stars in Tau-Aur; at 2 μm, it and the B9 star are equally bright. We see evidence of radial velocity variability in the cool component of over 34 km s-1. It is not visible in K-band speckle imaging, which constrains the companion to lie within 14 AU of the B star. We also report on a possible fourth member of the group, an M1 star 18″ south of HD 28867.