Hot white dwarfs and the UV delay in dwarf novae

We calculate the effect of illumination of dwarf nova accretion discs by ra diation from a hot, central, white dwarf. We show that, only for very hot w hite dwarfs (T-eff approximate to 40 000 K), the inner regions of quiescent dwarf nova discs are partially depleted so that the delay between the rise to outburst of the optical and UV fluxes would be increased as suggested r ecently by King. This depletion, however, must create several small outburs ts between the main outbursts, contrary to observations. Lower white dwarf temperatures may cause the outbursts to be of the 'inside-out' type, thus r emoving the UV delay. We conclude that white dwarf irradiation of dwarf nov a discs is not very efficient because, for example, the UV radiation from t he hot white dwarf does not penetrate deep enough in the disc atmosphere. T he total ablation of the inner disc by, e.g., evaporation (possibly related to illumination) appears to be a very promising possibility, accounting fo r both the extreme UV (EUV) delay and the general properties of the light c urves.