Abstract. 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 × 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 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 . The photometric measurements are analysed using a sophisticated light curve synthesis program and yield the following results: i = 84.6• ± 1.1• , R wd = 0.0189 ± 0.0010 R , and R Mstar (polar) = 0.174 ± 0.009 R . 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.

Abstract. We present the results of a combined analysis of low-resolution spectroscopy and high-speed optical photometry of the bright eclipsing polar HU Aqr, obtained in its high accretion state. Cyclotron harmonic emission in a field of B 34 MG became evident and could be traced through the whole optical bright phase. The cyclotron harmonics display an asymmetric motion as a function of phase which requires a certain forward tilt of the magnetic field lines in the emission region. The optical bright phase is more extended than the bright phase in soft X-rays suggesting a more extended emission region. Also, ingress into and egress from the optical eclipse last much longer in the optical than at X-ray wavelengths. These observations require the existence of an accretion arc with total extent of about 20• . The soft X-ray accretion spot is located at the far end of the arc, where the bulk of matter is accreted. The arc is likely to be more extended than the foot-line of field lines connecting to the ballistic accretion stream in a dipolar geometry. This together with the required tilt of the accreting field lines suggest a more complex magnetic geometry than a simple dipole.

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