ROSAT constraints on the intermediate polar candidates V426 Oph, SW UMa and 1H 0709-360.

ROSA T 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 ROSA T 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. Key words: stars: individual: V426 Oph - stars: individual: SW UMa - stars: individual: 1H0709 -360 - novae, cataclysmic variables - X-rays: stars.