Ultraviolet and optical observations of the dwarf novae VW and WX Hydri during outburst

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Abstract
We present simultaneous spectrophotometry in the range 1200–7000 Å, of the dwarf novae at quiescence and maximum. We compare the continuum spectra with the standard model disc spectrum and, with one exception, the fit is poor. The assumptions that the disc is both steady-state and blackbody are reconsidered and we point out that a ν1/3 spectrum is not to be expected from a disc sufficiently small to be accommodated within these short binary period systems. On the rise to outburst the UV flux in VW Hydri is observed to lag at least one day behind the optical, supporting the mass accretion event explanation of dwarf nova outbursts. The behaviour of the UV and optical line features during outburst is described, in particular, the P Cygni features observed in a super-outburst of WX Hydri, from which we estimate a mass loss rate, small compared with the mass transfer rate from the secondary star.

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