The variability in the UV resonance lines of the cataclysmic variables MU Cen, AH Her and QU Car
We present a time-variability study of the uV resonance lines Nv 1240, Si iv 1397 and C iv 1549 in the dwarf novae AH Her and MU Cen and the nova-like variable QU Car. The main findings are as follows. (i)Virtually no variation can be seen in our data for MU Cen. (ii) AH Her shows velocity shifting of its resonance lines, with semi-amplitudes ranging from approximately 500 to 800 km s1. Since the shifts appear to be linked to binary phase, this has been checked using a quantitative period-search procedure. In this procedure, three different techniques were used: the Lomb-Scargle periodogram, phase-dispersion minimization, and 2 minimization. Based primarily on comparisons with simulated data and a periodogram statistic due to Fisher, we find that we can significantly detect the orbital period in the velocity shifts. (iii) QU Car exhibits short-time-scale variations with a possible orbital trend in its uV resonance lines and/or continuum, but our data are insufficient to confirm the orbital nature of the variations. It is argued, on general grounds, that orbital-phase-linked variability of the UV resonance lines is more easily accounted for if it is due to departures from axisymmetry of the UV line emission from the accretion disc. The amplitudes of the velocity shifts suggest an inner disc location for this asymmetry in AH Her. The possibility of a secondary (uV) hotspot due to the interaction of the mass-transfer stream with the inner disc is considered.