A spectroscopic and evolutionary study of the pre-cataclysmic variable NN-Serpentis

Phase-resolved spectra of NN Ser taken in the blue and far red are presented. The spectra show Balmer lines in absorption, characteristic of hot white dwarf stars, and phase-dependent narrow single-peaked Balmer, He I and Ca II lines in emission, which we interpret as reprocessed light coming from the illuminated hemisphere of the red dwarf. We classify the primary as a DAO1 white dwarf, with an age in the range 10⁶ – 10⁷ yr. A cross-correlation technique was applied to find the radial velocities of the emission features. We find the semi-amplitude of the radial velocity curve for the Hα line to be K₂ = 310±10 km s^–1. Based on this and previous observational data, we derive a complete set of system parameters for NN Ser (Table 4). A study of the reflection effect on the secondary star shows symmetric heating only on the surface directly facing the primary star. An initial estimate of the temperature of the illuminated part yields 5650 < $$[T_{{\rm{i}}1}^*]$$ < 8150 K. The evolutionary implications of our results based on current theoretical stellar models make it probable that this system will show cataclysmic activity in about 2.2 × 10⁹ yr.