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ROSAT observations of three cataclysmic variables with disputed intermediate polar (DQ Her star) designations are presented. Pronounced variability, 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 during the early part of the observation. The previously reported orbital and rotational X-ray/optical periodicities in SW UMa were not detected in the ROSAT observations of this star: we place an upper limit of 20 per cent on the pulse fraction of any rotational modulation. Spectral analysis indicates that a two-temperature origin for the X-ray emission is probably appropriate. An X-ray imaging detection of the HEAO-1 source, 1H0709 – 360, is also presented. The weak signal precludes a detailed spectral analysis, but we are able to place limits on the column to the source. When extrapolated to the 2–10 keV band, the constraints 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 intermediate polar classification of each of the three sources remains unconfirmed.Peer-reviewedPublisher Versio

ROSAT constraints on the intermediate polar candidates V 426 Oph, SW UMa and 1H0709 - 360