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

We carried out optical observations of the field of the X-ray pulsator RX J0806.3+1527. A blue V = 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 (v similar to 1500 km s(-1)), low equivalent width (similar to -1 divided by -6 Angstrom) emission lines from the HeII Pickering series were clearly detected. B, V and R time-resolved photometry revealed the presence of similar to15% pulsations at the similar to321 s X-ray period, confirming the identification. These findings, together with the period stability and absence of any additional modulation in the 1 min-5 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.