Evidence for a brown dwarf in the TOAD V592 Herculis

We present optical R and I photometry of V592 Her obtained one year before its 1998 outburst. We identify V592 Her with a faint blue star, showing var iability in R with a standard deviation of similar to 0.07 mag. With mean q uiescent magnitudes of R = 21.5 and I = 21.4, its outburst amplitude is sim ilar to 10 mag, which is at the very upper end of the range observed for Tr emendous-Outburst-Amplitude Dwarf novae. A main sequence secondary would im ply a distance > 1500 pc, which is inconsistent with the absolute magnitude s in outburst and quiescence expected for a dwarf nova. We conclude that th e secondary is a brown dwarf. The quiescent flux is almost completely from the white dwarf, which gives a white dwarf temperature of 10 000 K and a di stance of similar to 700 pc. The non-detection in a ROSAT PSPC observation implies an upper limit to the Xray luminosity of less than or similar to 4 x 10(30) erg s(-1) and to the accretion rate onto the white dwarf of less t han or similar to 10(-12) M.yr(-1).