RE-1816- A NORTHERN-HEMISPHERE ANALOG TO THE LOW-MASS RAPID ROTATOR HK-AQR(541 )

Spectroscopic observations of the optical counterpart to the ROSAT Wid e Field Camera source RE 1816 + 541 are presented. We find that the co unterpart is a chromospherically active, rapidly rotating, dM1-2e star with a v sin i of 61 km s(-1). No evidence for radial velocity variat ions is found, and we conclude that RE 1816 + 541 is an example of the rare class of single, rapidly rotating field stars, and that it is si milar to the southern hemisphere object HK Aqr. assuming a main-sequen ce radius, a rotation period of 12 sin i hours is predicted. In common with several other active, rapid rotators, RE 1816 + 541 has a radial velocity consistent with membership of the Local Association, a comov ing group of young B stars and open clusters such as the Pleiades. On the basis of this and its rapid rotation, we suspect a similar youthfu l evolutionary status for RE 1816 + 541, making it a good candidate fo r studying angular momentum loss and extreme magnetic activity in youn g, low-mass stars.