The Hot Components in the Recurrent Novae T Pyxidis, IM Norma, CI Aquilae and in the Dwarf Nova U Geminorum

Abstract

We report on the latest results from Hubble Space Telescope (HST) far ultraviolet spectroscopy with the Cosmic Object Spectrograph (COS), of the short orbital period recurrent novae (RNe) T Pyxidis, IM Norma (the first ever FUV spectrum of this RN) and CI Aquilae, the only known recurrent novae with main sequence donors and orbital periods less than a day. The last two HST COS spectra we obtained in October, 2016 and June, 2016, reveal that the accretion rate of T Pyx has declined by 20% below its quiescent continuum flux level recorded by the IUE following the 1966 outburst, an indication that the system might not have yet reached its (new) quiescent state. Recent HST COS Spectroscopy of the cooling of the white dwarf in the prototypical dwarf nova U Geminorum is also discussed with a focus on the chemical abundances in the white dwarf photosphere.