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The system RX J0806.3+1527 (HM Cnc) is a pulsating X-ray source with 100 percent modulation on a period of 321.5 s (5.4 min). This period reflects theorbital motion of a close binary consisting of two interacting white dwarfs.Here we present a series of simultaneous X-ray (0.2-10 keV) andnear-ultraviolet (2600 angstrom and 1928 angstrom) observations that werecarried out with the Swift satellite. In the near-ultraviolet, the counterpartof RX J0806.3+1527 was detected at flux densities consistent with a blackbodywith temperature 27E+3 K. We found that the emission at 2600 angstrom ismodulated at the 321.5-s period with the peak ahead of the X-ray one by 0.28cycles and is coincident within 0.05 cycles with the optical. This phase-shiftmeasurement confirms that the X-ray hot spot (located on the primary whitedwarf) is at about 80-100 degrees from the direction that connects the twowhite dwarfs. Albeit at lower significance, the 321.5-s signature is presentalso in the 1928-angstrom data; at this wavelength, however, the pulse peak isbetter aligned with that observed at X-rays. We use the constraints on thesource luminosity and the geometry of the emitting regions to discuss themerits and limits of the main models for RX J0806.3+1527.

Swift X-ray and ultraviolet observations of the shortest orbital period double-degenerate system RX J0806.3+1527 (HM Cnc)