We have discovered a new AM Her system as the optical counterpart of the ROSAT All-Sky-Survey source RX J1016.9–4103 ( \equiv 1RXS J101659.4-410332 ) .
The X-ray spectrum is very soft and the X-ray intensity is strongly modulated with the orbital period .
Optical photometric and spectroscopic follow-up observations reveal a synchronously rotating binary with an orbital period of 134 min , placing RX J1016.9–4103 in the period gap .
The strength of the TiO bands suggests a secondary spectral type later than M3 V and a distance of 615 \pm 150 pc .
Based on two clearly visible broad humps in the optical spectrum ( interpreted as cyclotron features ) a magnetic field strength of 52 MG is deduced thus proving the polar classification .