EKTrA was selected as a target for our HST program as it appears to be very similar to the well-studied SUUMa dwarf nova VWHyi (Gänsicke et al. 1997). Our analysis of the STIS data confirms this suggestion: we find a white dwarf temperature and rotation rate that are very close to the values derived for VWHyi.
EKTrA is only the fifth CV white dwarf whose rotation rate could be accurately measured from the Doppler-broadened metal lines in high-resolution ultraviolet spectra, and it is only the second typical SUUMa-type system (Table 2). The other four stars include the ultra-short period large-outburst amplitude dwarf nova WZSge, the prototypical SUUMa dwarf nova VWHyi, and the two long-period UGem-type dwarf novae UGem and RXAnd. If we consider the membership to one of these three groups as a measure of the evolutionary stage of a system, with the UGem type stars above the orbital period gap being the youngest systems, the SUUMa stars below the gap being significantly older, and the WZSge stars being the oldest - possibly containing already degenerate donors and evolving to longer orbital periods - then the presently known white dwarf rotation rates are not in disagreement with an increase of the rotation rate with increasing age. Such a trend is indeed expected, as the angular momentum of accreted matter spins the white dwarf up (King et al. 1991), even though the long-term angular momentum evolution of accreting white dwarfs - taking into account nova explosions - is not yet well understood (Livio & Pringle 1998).
Acknowledgements
We thank the CTIO Director's Office for the allocation of discretionary time used for this project. CTIO is operated by AURA, Inc., under cooperative agreement with the United States National Science Foundation. BTG was supported by the DLR under grant 50OR99036. PS, EMS, and SBH acknowledge partial support of this research from HST grant GO-08103.03-97A.
Copyright ESO 2001