  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.
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. 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.
