THE ROSAT BRIGHT SOURCE RXJ0222.4- AN ACTIVE NEARBY SHORT-PERIOD BINARY OF THE BY DRACONIS TYPE(4729 )

We report the discovery of a new BY Dra-type binary identified as the optical counterpart of the bright source RX J0222.4+4729 detected duri ng the ROSAT All-Sky Survey (Voges et al. 1996). The star is a V simil ar to 11.1, near-by (similar to 30 pc), close spectroscopic binary wit h an orbital period P = 0.46543 +/- 0.00001 d. The absorption-line rad ial velocities were obtained at the 1.93-m Haute-Provence (OHP) telesc ope with the Elodie echelle spectrograph by on-line numerical crosscor relation. The M0Ve primary exhibits strong Balmer and Ca II H+K line e mission, placing this system amongst the most active BY Dra stars. The width of the cross-correlation function yields a projected rotational velocity of v sin i similar to 85 km/s. While only the primary contri butes to the continuum and the absorption line spectrum, the dM5e seco ndary is detected through its H alpha emission. The mass ratio, estima ted from the amplitudes of the emission radial velocity curves, is q = M-2/M-1 similar to 0.4. CCD photometry in the B and V bands, obtained with the OHP 1.2-m and 0.80-m telescopes, shows that the optical flux is modulated at the spectroscopic period with a total amplitude of 0. 2 mag and little or no color change in B - V. The light curve, which c an be attributed to rotational modulation of the synchronized active p rimary star, shows extrema near quadratures and also exhibits long-ter m variations in average brightness (by 0.1 mag), which are accompanied by changes around the photometric minimum. A secondary minimum appear s at phase 0.5. indicating a partial eclipse of the primary star. In c ontrast with many other BY Dra systems, the equivalent width of the H alpha emission from the RX J0222.4+4729 primary is directly correlated with photospheric brightness, i.e. maxima and minima occur around the same phases in both curves. However, the minimum at mid-phase in the H alpha equivalent width is broader and deeper than the V-band minimum at phi = 0.5 and appears shifted towards phase 0.45, suggesting that Ha emission comes from extended regions connecting the main starspot g roups. We find an X-ray to bolometric luminosity ratio of log(L-x/L-bo l) similar to -3.1 +/- 0.14 which supports the concept of saturation o f coronal X-ray emission for the most rapidly rotating late-type stars .