Letter to the Editor

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

¹ INAF – Osservatorio Astronomico di Roma, V. Frascati 33, 00040 Monteporzio Catone, Italy
² European Southern Observatory, Karl–Schwarzschildstr. 2, 85748 Garching, Germany
³ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
⁴ Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁵ Institut für Physik, Universitäts-Sternwarte München, Scheinerstr. 1, 81673 München, Germany
⁶ Subaru Telescope, NAOJ, 650 North A'ohoku Place, Hilo, Hawaii, HI 96720, USA
⁷ European Southern Observatory, Casilla 19001, Santiago, Chile
⁸ Lawrence Livermore National Laboratory, L–43, 7000 East Avenue, Livermore, CA 94550, USA
⁹ INAF – Osservatorio Astronomico di Padova, Sede di Asiago, 36012 Asiago, Italy
¹⁰ Observatoire Astronomique de Strasbourg, 11 rue de l'Université, 67000 Strasbourg, France
¹¹ Universitätssternwarte Göttingen, Geismarlandstr. 11, 37083 Göttingen, Germany
¹² INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025, Pino Torinese (To), Italy

Corresponding author: G. L. Israel, gianluca@mporzio.astro.it

Received: 12 February 2002
Accepted: 1 March 2002

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.