The long period intermediate polar 1RXS J154814.5-452845

We present the first time resolved medium resolution optical spectroscopy of the recently identified peculiar Intermediate Polar (IP) 1RXS J154814.5-452845, which allows us to precisely determine the binary orbital period (P-Omega=9.87 +/- 0.03 h) and the white dwarf spin period (P-omega=693.01 +/- 0.06 s). This system is then the third just outside the purported similar to 6-10 h IP orbital period gap and the fifth of the small group of long period IPs, which has a relatively high degree of asynchronism. From the presence of weak red absorption features, we identify the secondary star with a spectral type K2 +/- 2 V, which appears to have evolved on the nuclear timescale. From the orbital radial velocities of emission and the red absorption lines a mass ratio q=0.65 +/- 0.12 is found. The masses of the components are estimated to be M-WD >= 0.5 M-circle dot and M-sec=0.4-0.79 M-circle dot and the binary inclination 25 degrees<i <= 58 degrees. A distance between 540-840 pc is estimated. At this distance, the presence of peculiar absorption features surrounding Balmer emissions cannot be due to the contribution of the white dwarf photosphere and their spin modulation suggests an origin in the magnetically confined accretion flow. The white dwarf is also not accreting at a particularly high rate ((M)over dot< 5x10(16) g s(-1)), for its orbital period. The spin-to-orbit period ratio P-omega/P-Omega=0.02 and the low mass accretion rate suggest that this system is far from spin equilibrium. The magnetic moment of the accreting white dwarf is found to be mu