X-ray Spectroscopy of Chemically Peculiar B Stars, or Their Unseen Companions Ehud Behar Technion Department of Physics Haifa, 32000 Israel Late type B stars are not supposed to emit X-rays. Their apparent detection by X-ray telescopes gives rise to the companion hypothesis. We will present spectroscopic methods for testing this hypothesis and dwell on the importance of the atomic data used, the methods for obtaining it, and on its accuracy. We will apply these methods to high-resolution Chandra and XMM-Newton observations of the chemically peculiar late-type B star mu Lep and show that, indeed, in this case the X-rays can be traced back to a previously unresolved companion (mu Lep B), whose X-ray spectrum resembles that of a coronal source. We will present evidence that supports mu Lep B being a pre-main-sequence star, most likely of the non-accreting magnetically-active type.