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 pre sented. The spectra show Balmer lines in absorption, characteristic of hot white dwarf stars, and phase-dependent narrow single-peaked Balme r, He I and Ca II lines in emission, which we interpret as reprocessed light coming from the illuminated hemisphere of the red dwarf. We cla ssify the primary as a DAO1 white dwarf, with an age in the range 10(6 )-10(7) yr. A cross-correlation technique was applied to find the radi al velocities of the emission features. We find the semi-amplitude of the radial velocity curve for the Halpha line to be K2 = 310 +/- 10 km s-1. Based on this and previous observational data, we derive a compl ete set of system parameters for NN Ser (Table 4). A study of the refl ection effect on the secondary star shows symmetric heating only on th e surface directly facing the primary star. An initial estimate of the temperature of the illuminated part yields 5650 < T(il) < 8150 K. Th e evolutionary implications of our results based on current theoretica l stellar models make it probable that this system will show cataclysm ic activity in about 2.2 x 10(9) yr.