cm$^{-2}$. This favours Case B recombination for which one would expect $F_{\lambda1640}/F_{\lambda4686}\sim7$ (Seaton 1978). Therefore, the region is optically-thick to the ionizing flux but thin to the line and a fraction $\epsilon\approx0.2$ of the photoionized He[i]{} recombinations lead to $\lambda$4686 emission (Hummer & Storey 1987).

Following previous applications to soft X-ray transients (Marsh, Robinson & Wood 1994; Hynes et al. 1998), the EUV flux $F_{\rm EUV}$ and the He[ii]{} line flux $F_{\lambda4686}$ are related through $$\epsilon\alpha\frac{F_{\rm EUV}}{E_{\rm EUV}}=\frac{F_{\lambda4686}}
{E_{\lambda4686}}$$ where $\alpha$ is the fraction of EUV photons intercepted by the disc while $E_{\rm EUV}\approx100$ eV and $E_{\lambda4686}\approx2.6$ eV refer to the mean energy of the photons. We have assumed $L_{\rm
EUV}/L_{\lambda4686}=F_{\rm EUV}/F_{\lambda4686}$, which may be incorrect if the EUV emission is not isotropic or if we see only a fraction of the He[ii]{} emission. In both cases this assumption leads to an underestimate of the true EUV luminosity. Observations suggest that a fraction $\sim 10^{-3}$ of the soft X-ray flux is intercepted and reprocessed in the optical by the accretion disk in SXTs (e.g. Dubus et al. 1999). By analogy, we take $\alpha=10^{-3}$ from which we finally derive [ an upper limit to the]{} 55-280 eV flux of $10^{-8}$ erg 