Multiwavelength monitoring of the dwarf nova VW Hydri – II. Walraven photometry⋆

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Abstract
In this paper we present results of five-colour photometric observations (Walraven system) of VW Hyi, made between 1984 July and November, as part of a long-term multiwavelength study of this dwarf nova. The average orbital light and colour curves during quiescence are accurately defined. Apart from a slow decay following outburst during the first quarter of a quiescent interval, neither the average brightness nor the maximum and minimum brightness show a significant secular trend between outbursts. The 3σ upper limits to a secular change in average brightness during the last three-quarters of a quiescent interval range between 0.16 and 0.20 mag in the five passbands. There is no strong evidence for a secular change in the shape of the orbital light curve (amplitude and width of the hump) through the quiescent intervals between outbursts. Between outbursts the colour behaviour of VW Hyi shows a significant variation. The system is substantially bluer just after outburst in the B–U colour, by about 0.09 mag. The shape of the orbital B–U and U–W colour curves change through the last three-quarters of a quiescent interval. In this part of quiescence the amplitude of the B–U colour curve decreases by 0.08±0.02mag, and that of the U–W colour curve increases by 0.14±0.05 mag. Over the wavelength range covered by our photometry the onset of normal outbursts is progressively delayed towards shorter wavelengths. The corresponding change in the spectral energy distribution during the rise of normal outbursts fits the behaviour observed with the International Ultraviolet Explorer.

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