The discovery of 321 s pulsations in the ROSAT HRI light curves of 1BMW J080622.8+152732=RX J0806.3+1527

During a systematic search for periodic signals in a sample of similar to 3 000 ROSAT HRI (0.1-2.4 keV) light curves, we discovered similar to 321 s pu lsations in the X-ray flux of RX J0806.3+1527. Two different HRI observatio ns of RX J0806.3+1527 were obtained with the source at flux level of 3 and 5 x 10(-12) erg cm(-2) s(-1), respectively. Only a faint B = 20.5 object is possibly present within the error circle, while no optical counterpart is present in the R plate down to a limiting magnitude of similar to 20. This indicates that the object is intrinsically blue. The X-ray and optical find ings imply that RX J0806.3+1527 is a relatively distant (similar to 500 pc) intermediate polar (IP) or, more unlikely, a nearby (similar to 10 pc) iso lated neutron star accreting from the interstellar medium.