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We report on the results of a spectroscopic and photometric study of a newcataclysmic variable, identified as optical counterpart of the X-ray source RXJ1554.2+2721 detected by ROSAT. The spectroscopic observations of therelatively bright (~16.5mag) object show systematic radial velocity variationswith a semi-amplitude of ~140 km/sec. Besides the clear presence of distinctlow and high states there are periodic photometric light variations with anamplitude of about 0.15 magnitude in the R band. The orbital period is 2.753hthus being within the period gap, at its upper border. The flux distribution inthe spectrum of the object shows a substantial contribution of a M4V secondary,and also bears clear signs of cyclotron emission. Thus, we classify thediscovered object as a new member of the AM Her class of magnetic cataclysmicvariables. This classification is further supported by the soft X-ray spectrum,the characteristic profiles of the emission lines, the tomography map and theshape of the orbital light curve. A simple fitting of the spectrum in the lowand high states suggests a reduced mass transfer rate in RX J1554.2+2721,compared to similar objects outside the period gap.Comment: 11 pages, 11 figures, accepted in A&

The new cataclysmic variable RX J1554.2+2721 in the period gap