Refined system parameters for the pre-cataclysmic binary NN Serpentis

NN Ser is known to be a 17 mag pre-cataclysmic binary consisting of a hot white dwarf and a cool late type star orbiting each other with a period of 3(h) 7(m). The system shows very deep eclipses and a pronounced reflection effect. Using the FORS instruments at the VLT the late type star could now be detected photometrically at 22.8 mag during eclipse and monitored spectroscopically. These data combined with earlier high speed photometric and phase-resolved spectroscopic observations form the basis for a determination of refined system parameters for NN Ser. The spectral type of the late type star is found to be M4.75. A model atmosphere analysis of the white dwarf yields a temperature of 57 000 +/- 3000 K and log g = 7.6 +/- 0.1. The presence of He in the atmosphere (N-He = 2 +/- 0.5 x 10(-4) by number) indicates that the white dwarf is a hydrogen-helium hybrid star of type DAO1. Since the derived radial velocity curves prevent an unambiguous determination of the mass ratio the white dwarf's mass of 0.54 +/- 0.05 M-circle dot is inferred using the results of the model atmosphere analysis and recent evolutionary models. The mass of the M star is determined via a well calibrated M-R relation to be 0.150 +/- 0.008 M-circle dot. The photometric measurements are analysed using a sophisticated light curve synthesis program and yield the following results: i = 84.6degrees +/- 1.1degrees, R-wd = 0.0189 +/- 0.0010 R-circle dot, and R-Mstar (polar) = 0.174 +/- 0.009 R-circle dot. The shape of the cool star turns out to be slightly ellipsoidal. Its temperature at the un-heated hemisphere (backside) is 2 920 +/- 70 K while the heated hemisphere (sub-stellar point) has a temperature of 7125 +/- 200 K.