4686$ line rest wavelength and appears consistent with the claimed photometric period of $4.1$ hr (Cook et al. 2000; Patterson 2000). The modulated component is particularly strong towards the middle of the observation at HJD 51654.876 where it is blueshifted by about 900 km s$^{-1}$.

The trailed spectrum clearly shows that the Bowen blend, He[i]{} $\lambda4921.9$, H$\gamma$ and H$\beta$ follow the same S-wave pattern (see Fig. \[fig:swave\] ). We note that identical behaviour is seen in the smaller continuous set (1.8 hr) of July 4 although with poorer S/N (not shown here). However, the presence of the S-wave in the H$\alpha$ emission profile is not clear on either dates.

Despite the incomplete phase coverage and the absence of a reference for superior conjunction of the secondary, we attempted to locate the emission site of the S-wave in the binary velocity plane using the Doppler tomography technique (Marsh & Horne 1988). Maps were reconstructed for He[ii]{} and H$\alpha$ for the April 20 observations using the software developed by Spruit (1998), assuming an orbital period of 0.1708 day and a null systemic velocity. It was not possible to combine this dataset with our other observations as this would have required $P_{\rm orb}$ 