Abstract
BE Ursae Majoris is a 15-mag variable star of period 2.29 days, with a sinusoidal light curve of ∼1 mag amplitude1. Recently, Ferguson et al.2 have drawn attention to this object, also known as PG1155 + 492, because of its UV excess and unusual spectrum, which they observed to vary between absorption and strong emission on a time scale of months. Here we report observations which demonstrate that the spectrum of BE UMa is dramatically variable on a time scale of hours and days, in synchrony with the 2.29-day periodic light variations. We interpret the system as a detached binary consisting of an extremely hot (T∼105K) white dwarf and a cool M star, whose upper layers are irradiated by the intense ionizing flux of the hot companion. This unique system should prove invaluable for measurements of the mass, luminosity and bolometric correction of the hottest white dwarfs.
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Margon, B., Downes, R. & Katz, J. Synchronous extreme spectral variability of BE Ursae Majoris. Nature 293, 200–202 (1981). https://doi.org/10.1038/293200a0
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DOI: https://doi.org/10.1038/293200a0
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