Phase-resolved spectroscopy and photometry of the eclipsing polar EP Draconis (=H1907+690)

Abstract

We present phase-resolved optical spectroscopy and CCD photometry of the faint eclipsing polar EP Dra (H1907+690). A revised ephemeris is derived which connects all 32000 binary cycles since its discovery by Remillard et al. (1991). We found no difference between spin and and orbital periods of the white dwarf. Changes in the light curve morphology are attributed to a different beaming behaviour which might change on timescales as short as one or several orbital periods. Optical light curve modelling was used to estimate the co-latitude of the accretion spot, which must be larger than 40 degrees. We have detected Zeeman absorption lines of Halpha originating in an accretion halo in a field of 16MG. The low-resolution spectra reveal no indication of resolved cyclotron harmonics, which is also suggestive of a relatively low field strength in the accretion region. The Balmer emission lines contain significant contributions from the UV-illuminated hemisphere of the companion star, whereas the HeII4686 emission originates predominantly from the accretion stream. The emission lines have a multi-component structure and we could single out a narrow emission line in the Hbeta and Hgamma lines. Its radial velocity amplitude suggests a low mass for the white dwarf, if the lines are interpreted as being of reprocessed origin from the whole illuminated hemisphere of the companion star.