RX J0806.3+1527: A double degenerate binary with the shortest known orbital period (321s)

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Abstract
We carried out optical observations of the field of the X–ray pulsator RX J0806.3+1527. A blue 21.1 star was found to be the only object consistent with the X–ray position. VLT FORS spectra revealed a blue continuum with no intrinsic absorption lines. Broad ( 1500 km s-1), low equivalent width (~–1 –6 Å) emission lines from the HeII Pickering series were clearly detected. B, V and R time–resolved photometry revealed the presence of ~15% pulsations at the ~321 s X–ray period, confirming the identification. These findings, together with the period stability and absence of any additional modulation in the 1 min5 hr period range, argue in favour of the orbital interpretation of the 321 s pulsations. The most likely scenario is thus that RX J0806.3+1527 is a double degenerate system of the AM CVn class. This would make RX J0806.3+1527 the shortest orbital period binary currently known and one of the best candidates for gravitational wave detection.
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