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 during the ROSAT All-Sky Survey ( Voges et al .
1996 ) .
The star is a V \sim 11.1 , near-by ( \sim 30 pc ) , close spectroscopic binary with an orbital period P = 0.46543 \pm 0.00001 d. The absorption-line radial velocities were obtained at the 1.93-m Haute-Provence ( OHP ) telescope with the Elodie echelle spectrograph by on-line numerical cross-correlation .

The M0Ve primary exhibits strong Balmer and Ca II H+K line emission , 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 \sim 85 km/s .
While only the primary contributes to the continuum and the absorption line spectrum , the dM5e secondary is detected through its H \alpha emission .
The mass ratio , estimated from the amplitudes of the emission radial velocity curves , is q = M _ { 2 } / M _ { 1 } \sim 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 can be attributed to rotational modulation of the synchronized active primary star , shows extrema near quadratures and also exhibits long-term variations in average brightness ( by 0.1 mag ) , which are accompanied by changes around the photometric minimum .
A secondary minimum appears at phase 0.5 , indicating a partial eclipse of the primary star .

In contrast 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 H \alpha emission comes from extended regions connecting the main starspot groups .

We find an X-ray to bolometric luminosity ratio of \log ( L _ { x } / L _ { bol } ) \sim - 3.1 \pm 0.14 which supports the concept of saturation of coronal X-ray emission for the most rapidly rotating late-type stars .