Astron. Astrophys. 352, 563-566 (1999)

1. Introduction

In many close binary systems the hotter star will heat one face of the cooler star. As the orbital motion of the binary brings this face into view the observed flux from the binary will increase, only to fall again as it rotates out of view. After a nova explosion, the hot white dwarf is an obvious candidate for heating its cool companion. Probably the best evidence is in V1500 Cyg (Nova Cyg 1975), where Schmidt et al. (1995) show that the secondary star dominates the photometric modulation. They fitted HST spectra with a red star whose unperturbed temperature was K but whose face towards the white dwarf was K.

There are three other novae for which there is photometric evidence for heating. The orbital modulation in DN Gem (Nova Gem 1912) was found by Retter, Leibowitz & Naylor (1999) to be well described by a heating model and DeYoung & Schmidt (1994) suggested heating could explain the lightcurve of V1974 Cyg (Nova Cyg 1992). Finally, Somers et al. (1996) found the infrared lightcurve of WY Sge (Nova Sge 1783) required a heated face to be modelled successfully. However, in this case the level of irradiation was so low that it could have been supplied by the accretion luminosity, as occurs in the dwarf nova IP Peg during outburst (Webb et al. 1999).

This led us to ask if there were further evidence available which would help us identify the source of irradiation in old novae as the white dwarf. It is particularly important to do so, as the irradiated surface of the secondary star may be the most reliable diagnostic we have of the white dwarf luminosity, since its intrinsic radiation is produced in the far UV/soft X-ray regimes, where the effect of interstellar absorption will be very marked. If the irradiating object is the white dwarf, the irradiation should decrease on the white dwarf cooling timescale. Prialnik (1986) shows how the surface layers of a white dwarf are heated during a nova explosion and cool as a power law on a time scale of 200 yrs. (This is in contrast to the cooling of white dwarfs after their initial formation, which involves cooling of the entire star and occurs on a time scale of years.)

© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999
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