An illumination effect and an eccentric orbit for the symbiotic binary PU Vul revealed by 32 years of optical spectroscopy

We analyze ∼32 years of optical spectra and photometry for the symbiotic binary PU Vul. Light curves for the He I λ4471, He II λ4686 and Hβ λ4861 emission lines reveal an illumination effect, where the hot white dwarf ionizes the outflowing wind of the red giant, and evidence for an eccentric orbit with e ≈ 0.16. Along with the gradual appearance of high ionization emission from [Fe VII] and O VI, the relative fluxes of these lines suggest an increase in the effective temperature of the hot component, from roughly 10^5 K on JD 2448000 (1990) to roughly 2× 10^5 K on JD 2455000 (2009). During this period, the luminosity of the hot component dropped by a factor of 4-6 to a current value of roughly 1000 L_⊙.