RE-0618- A VERY SHORT-PERIOD, BINARY DME SYSTEM(75 )

We report on the discovery of a new, very short-period, chromospherica lly active, late-type binary system. Observations performed with the R OSAT Wide Field Camera revealed the EUV source RE 0618 + 75, and subse quent optical spectroscopy identified this source as a chromospherical ly active, double dMe star with intense Ca II H and K and Halpha emiss ion lines. Further spectroscopy revealed that the object is a binary s ystem with a period of 0.539 d, a mass ratio of unity and a systemic v elocity of -14 km s-1. Spectral types of M 3 for both components are d educed from low-resolution spectra, and this yields a system inclinati on of 34-degrees and component separation of 2.4 R.. RE 0618 + 75 is t he most rapidly rotating, binary dMe system known, yet the magnetic ac tivity observed is no greater than many similar, but more slowly rotat ing, dMe stars. This reinforces the concept of activity saturation in rapidly rotating stars.