2.3.4. Detached Eclipsing Binaries

Detached double-spectroscopic eclipsing binaries provide us with a unique chance to obtain the distance in a semi-geometric way out to LMC or even farther. From the information given by the light curve and velocity curve, one can solve for the orbital parameters and stellar radii (Andersen 1991, Paczynski 1997; Bell et al. 1993 for an earlier application to LMC HV2226; Torres et al. 1997 for an application to the Hyades). If surface brightness of the two stars is known from colour or spectrum, one can obtain the distance as d = (F/f)^1/2R_i where F and f are fluxes at the source and the observer and R_i is stellar radius. Guinan et al. (1998) applied this method to HV2274 in LMC and derived m - M = 18.30 ± 0.07 with the aid of Kurucz' model atmosphere to estimate surface brightness from the spectrum. Udalski et al. (1998b) claimed that the extinction used is too small by an amount of E(B - V) = 0.037 mag based on OGLE multicolour photometry. If we accept this correction the distance becomes 0.11 mag shorter, i.e., m - M = 18.19.