Is 2RE J0357+283 the most coronally active star in the Galaxy?

The late-type stellar counterpart to the EUV source 2RE J0357+283 is found to be an ultra-fast rotating K2V star, with a rotation period of 8.76 hrs and vsini of 141+/-5km/s. If this were the only source of the EUV emission, then it would be the most coronally active star known, by an order of magnitude, and requires a reappraisal of current ideas on dynamo saturation. However, the chromospheric activity appears no larger than would be expected for such a fast rotating star, UV spectroscopy reveals a probable white-dwarf companion and X-rays are only detected in the 0.1-0.4keV range. A plausible explanation for the UV, EUV and X-ray emission is found for an accompanying white-dwarf of temperature about 35000K. The rapid rotation of the cool star is puzzling. It is not in a close binary system and neither is it likely to be very young. We speculate that accretion of the wind from the giant progenitor of the white-dwarf in a detached system may transfer the required angular momentum, and that the cool component of 2RE J0357+283 is one of a new class of active star.