ROSAT CONSTRAINTS ON THE INTERMEDIATE POLAR CANDIDATES V426 OPH, SW UMA AND 1H0709-360

ROSA T observations of three cataclysmic variables with disputed inter mediate polar (DQ Her star) designations are presented. Pronounced var iability, with possible time-scales of 12.5, 2.5 or 4.5 h, is seen in the X-ray light curve of V426 Oph; these variations may be accompanied by spectral changes. This system may have been caught in the decline from a dwarf nova outburst, being significantly brighter and softer du ring the early part of the observation. The previously reported orbita l and rotational X-ray/optical periodicities in SW UMa were not detect ed in the ROSAT observations of this star: we place an upper limit of 20 per cent on the pulse fraction of any rotational modulation. Spectr al analysis indicates that a two-temperature origin for the X-ray emis sion is probably appropriate. An X-ray imaging detection of the HEAO-1 source, 1H0709-360, is also presented. The weak signal precludes a de tailed spectral analysis, but we are able to place limits on the colum n to the source. When extrapolated to the 2-10 keV band, the constrain ts imposed by the ROSAT data yield a flux that is about two orders of magnitude fainter than that implied by the HEAO-1 detection. The inter mediate polar classification of each of the three sources remains unco nfirmed.