Context : This is the fifth paper in a series aimed at studying the chromospheres of active binary systems using several optical spectroscopic indicators to obtain or improve orbital solution and fundamental stellar parameters . Aims : We present here the study of FF~UMa ( 2RE~J0933+624 ) , a recently discovered , X-ray/EUV selected , active binary with strong H \alpha emission . The objectives of this work are , to find orbital solutions and define stellar parameters from precise radial velocities and carry out an extensive study of the optical indicators of chromospheric activity . Methods : We obtained high resolution echelle spectroscopic observations during five observing runs from 1998 to 2004 . We found radial velocities by cross correlation with radial velocity standard stars to achieve the best orbital solution . We also measured rotational velocity by cross-correlation techniques and have studied the kinematic by galactic space-velocity components ( U , V , W ) and Eggen criteria . Finally , we have determined the chromospheric contribution in optical spectroscopic indicators , from Ca ii H \& K to Ca ii IRT lines , using the spectral subtraction technique . Results : We have found that this system presents an orbital period variation , higher than previously detected in other RS CVn systems . We determined an improved orbital solution , finding a circular orbit with a period of 3.274 days . We derived the stellar parameters , confirming the subgiant nature of the primary component ( M _ { P } = 1.67 M _ { \odot } and R \sin { i } _ { P } = 2.17 R _ { \odot } ) and obtained rotational velocities ( v \sin { i } ) , of 33.57 \pm 0.45 km s ^ { -1 } and 32.38 \pm 0.75 km s ^ { -1 } for the primary and secondary components respectively . From our kinematic study , we can deduce its membership to the Castor moving group . Finally , the activity study has given us a better understanding of the possible mechanisms that produce the orbital period variation . Conclusions :