4 Discussion

According to Morales-Rueda & Marsh (2002), the presence of He II 4686 emission during high states could be the result of disc irradiation or emission by spiral shocks structures. Therefore, the detection of He II 4686 in V 592 Her could indicate also the presence of spiral shocks in this short orbital period system. Supporting this view is the fact that Baba et al. (2002) imaged the WZ Sge disk during its 2001 superoutburst using the He II 4686 line finding arc-like structures. Higher quality data than currently available are necessary to confirm this suspicion in the case of V 592 Herculis.

The period excess is defined as:

$$\epsilon = \frac{P_{\rm s}-P_{\rm o}}{P_{\rm o}}$$ (2)

where $P_{\rm o}$ is the orbital period of the binary and $P_{\rm s}$is the superhump period. The period excess is an important observational parameter in the theory of disk tidal instability, since it can be related to the ratio between the stellar masses q = M₂/M₁:

$$1/\epsilon = [0.37q/(1+q)^{1/2}]^{-1}(R_{\rm disk}/0.46a)^{-2.3}-1$$ (3)

where a is the binary separation (Patterson 2001). Here we use Patterson's approximation:

$$\epsilon = 0.216(\pm0.018)q.$$ (4)

Using the orbital period $P_{2} = 85.5 \pm 0.4$ min, along with the superhump period given by Duerbeck & Mennickent (1998), we calculate a period excess of $0.011 \pm 0.006$, one of the shorter among SU UMa stars (Patterson 2001). This period excess implies a mass ratio $q = 0.05 \pm 0.03$. If the white dwarf has a typical mass of $0.7~M_\odot$, we find $M_{2} = 0.035 \pm 0.021~M_{\odot}$. Comparing this value with the Kumar limit for Population I stars ($\approx$ $0.07~M_\odot$, Kumar 1963) we find that the secondary could be a brown dwarf like object. Only if the white dwarf is massive ($\ga$$1~M_\odot$) is the mass ratio consistent with a non-degenerate hydrogen-burning star. On the other hand, if P₁ (91.2 min) is the right period, we obtain $\epsilon = 0.0096 \pm 0.0070$ and $q = 0.04 \pm 0.03$. The above shows that our result of a possible brown-dwarf like secondary in V 592 Her is robust against a misidentification of the orbital period. In addition, this finding in agreement with the result of van Teeseling et al.  (1999), who arrived at this conclusion using considerations about magnitudes at outburst and quiescence only.