We report the discovery of a new AM Herculis binary ( polar ) as the optical counterpart of the soft X-ray source RX J1724.0+4114 detected during the ROSAT all-sky survey .
The magnetic nature of this V \sim 17 ^ { m } object is confirmed by low-resolution spectroscopy showing strong Balmer and HeII emission lines superimposed on a blue continuum which is deeply modulated by cyclotron humps .
The inferred magnetic field strength is 50 \pm 4 MG ( or possibly even \approx 70 MG ) .
Photometric observations spanning \sim 3 years reveal a period of 119.9 min , right below the period gap .
The morphology of the optical and X-ray light curves which do not show eclipses by the secondary star , suggests a self-eclipsing geometry .
We derive a lower limit on the distance of d \mathrel { \hbox { \raise 2.15 pt \hbox { $ > $ } \hbox to 0.0 pt { \lower 2.15 pt \hbox { $ \sim% $ } } } } 250 pc .