YY Draconis and V709 Cassiopeiae: two intermediate polars with weak magnetic fields

We present data from long ROSATHRI observations of the intermediate polars YY Dra and V709 Cas which show that V709 Gas, like YY Dra, exhibits a doubl e-peaked X-ray pulse profile. Neither system shows evidence for X-ray beat period or orbital modulation, so both must be disc-fed accretors seen at lo w inclination angles. We argue that the short spin periods of the white dwa rfs in these objects indicate that they have weak magnetic fields, so the r adius at which material is captured by the field lines is relatively small. Consequently the footprints of the disc-fed accretion curtains on the whit e dwarf surface are large. The optical depths to X-ray emission within the accretion curtains are therefore lowest in the direction along the magnetic field lines, and highest in the direction parallel to the white dwarf surf ace, such that the emission from the two poles conspires to produce double- peaked X-ray pulse profiles. We emphasise that such a pulse profile is not a unique indicator of two-pole accretion however. Indeed, two-pole accretio n onto smaller regions of the white dwarf surface may be considered the 'no rmal' mode of behaviour in a disc-fed intermediate polar with a longer whit e dwarf spin period (and therefore a higher field strength), resulting in a single-peaked pulse profile. Collating data on other intermediate polars, we may classify them into two subsets. Fast rotators, with relatively weak fields, show double-peaked pul se profiles (AE Aqr, DQ Her, XY Ari, CK Per, V709 Gas, YY Dra, V405 Aur), w hilst slower rotators, with larger fields and therefore larger magnetospher es, have been seen to exhibit an X-ray beat period modulation at some time (FO Aqr, TX Col, BG CMi, AO Psc, V1223 Sgr, RX J1712.6-2414).