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Time-resolved spectroscopy of the pulsating CV GW Lib

We present time-resolved optical spectroscopy of the dwarf nova GW Librae during its rare April 2007 super-outburst and compare these with quiescent epochs. The data provide the first opportunity to track the evolution of the principal spectral features. In the early stages of the outburst, the optically thick disc dominates the optical and the line components show clear orbital radial velocity excursions. In the course of several weeks, optically thin regions become more prominent as strong emission lines replace the broad disc absorption. Post-outburst spectroscopy covering the I-band illustrates the advantages of CaII relative to the commonly used Balmer lines when attempting to constrain binary parameters. Due to the lower ionisation energy combined with smaller thermal and shear broadening of these lines, a sharp emission component is seen to be moving in between the accretion disc peaks in the CaII line. No such component is visible in the Balmer lines. We interpret this as an emission component originating on the hitherto unseen mass donor star. While our conservative dynamical limits place a hard upper limit on the binary mass ratio of q<0.23, we favour a significantly lower value near q~0.06. Pulsation modeling suggests a WD mass ~1 M_sun. This, paired with a low mass donor, near the empirical sequence of an evolved CV close to the period bounce, appears to be consistent with all the observational constraints to date. [abridged]