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We report on the first near-simultaneous X-ray and optical observations of RXJ0806.3+1527. The source is believed to be a 321s orbital period ultra-compactbinary system hosting an X-ray emitting white dwarf. Data were obtained withChandra and the ESO Very Large Telescope (VLT) in November 2001. We found anoptical/X-ray phase-shift in the periodic modulation of about 0.5, stronglyfavoring the existance of two distinct emission regions in the two bands (forthe pulsed fluxes). The Chandra data allow us to study, for the first time, thespectral continuum of RX J0806.3+1527 in soft X-rays. This was well fitted by ablackbody spectrum with kT about 65eV and hydrogen column density of N_H about5 x 10^20 cm^-2. The average (unabsorbed) source 0.1-2.5 keV luminosity duringthe modulation-on is L_X about 5 x 10^32 erg s^-1 (assuming a distance of500pc). Such a value is lower than the luminosity expected if stable masstransfer between two white dwarfs were driven by gravitational radiation.Evidence for absorption-like features are present in the phase-averagedspectrum at about 0.53, 0.64, and 1.26 keV, which may be attributed to heavyelements (C and N). We compare and discuss these findings with other binarysystems hosting an accreting white dwarf.Comment: Accepted for publication on ApJ. 8 pages (including 8 figures

Unveiling the Nature of the 321 Second Modulation in RX J0806.3+1527: Near‐SimultaneousChandraand Very Large Telescope Observations