Multiwavelength monitoring of QS Tel

Multiwavelength monitoring observations of the AM Her system, QS Tel, are p resented, the centrepiece being a coordinated campaign with the ASCA, EUVE and HST satellites and optical observations on 1996 September 28-29, The st ar was initially observed in an intermediate brightness, single pole state in 1996 April by ROSAT. However two EWE snapshots of the system in August f ound it in a deep low state, the light curves appearing to retain a bright and faint phase morphology although the bright interval showed significant phase drifts compared with the earlier ROSAT data. At this time, the system appeared optically bright, while polarimetry pointed to one active pole. A modest recovery of intensity, albeit accompanied by an apparently chaotic extreme ultraviolet (EUV) light curve, was subsequently recorded during the coordinated run in late September. This brightening was confirmed by a sec ond ROSAT pointing in early October, although the EW brightening is matched by a marked decay in its optical output. These observations, all of which found the system in a fainter EUV state than seen in 1993, add to the catal ogue of accretion mode behaviour in QS Tel. However, as yet, the data base of observations does not allow us to establish whether the accretion mode i s dictated by accretion-rate dependent penetration of the magnetosphere or asynchronous rotation. The ASCA spectrum obtained during the coordinated observation is adequately described by a thermal spectrum with a temperature of about 4.4 keV and an absorbing column density of less than 8 x 10(20) cm(-2). At the same time, the 1150-2600 Angstrom UV HST spectrum shows a strong, broad Ly alpha abso rption profile which we attribute to the white dwarf. We infer an effective temperature of 16 000-19 000 K and a mass between 0.5 and 0.93 M. for the white dwarf, at least some of the uncertainty arising because the profile i s very probably contaminated and broadened by Zeeman features. The orbital variation seen by HST can be matched by a blackbody-like spectrum with a te mperature above 24 000 K. The near UV part of the spectrum contains evidenc e of phase-variable cyclotron harmonics.