The white dwarf revealed in the intermediate polar V709 Cassiopeiae

Results are presented from the first detailed spectroscopic observations of the recently identified intermediate polar RX J0028.8+5917/V709 Cas. The s tudy of the emission line radial velocities allows us to remove the uncerta inties on the different aliases of the orbital period and a best value is f ound at (0.2225 +/- 0.0002) day. It is also found that the system shows sig nificant EW similar to (2-4) Angstrom broad absorptions affecting the Balme r lines from H-delta to H-beta. These broad absorptions are interpreted as the contribution of an underlying white dwarf atmosphere. The characteristi cs of the absorptions are found to be consistent with a DA log g = 8 white dwarf at a temperature of similar to 23 000 K, contributing similar to 17% (at 4500 Angstrom) to the overall flux. This is the first direct detection of a white dwarf in an intermediate polar system. The absence of significan t Zeeman splitting indicates a magnetic field lower than 10 MG, confirming that, at least in some cases, intermediate polars have weaker fields than p olars. Different possibilities are discussed to explain the substantial con tribution of the white dwarf to the overall flux.