This table, for various passbands u with effective wavelength A.eff(u) measured in nm gives the absolute flux values for zero apparent magnitude, e.g., /J(V) = 0.378 erg/cm3/s is the Johnson V flux for mV = 0. The conversion from arbitrary apparent magnitude mV to absolute flux /(mV) is then given by

Scaling using both calibrations differs by only 4% which seems to be sufficiently accurate given that the error in the distance varies with the square root, e.g., for Johnson V with V 1.04 « 1.02, or V0.975 ^ 0.98, respectively. In addition to the Johnson or Bessell calibration one needs to know the interstellar extinction A in magnitudes.

A nonstandard or non-calibrated light curve essentially contains information about the components' surface brightness via the ratio of eclipse depths yielding a relation

As this provides no further temperature information, we have one relation for two unknown temperatures. Therefore, it has been common practice to adopt one temperature, usually T1, from spectra or color indices and solve for the other one. Following Wilson (2007a, b) one can also understand why additional light curves in other passbands do not help to determine both temperatures. In an ideal situation

2 Standardized means U, B, V, etc. - not only on a standard system but with magnitudes as opposed to magnitude differences. In astronomical photometry, this is referred to as "absolute photometry."

where all system parameters are known and the radiative emission is described fully and correctly, the photometry and spectroscopy are reproduced correctly - only the pair (71, 72) is not yet resolved. Theory then will correctly predict T2 for given T1, for every passband from the primary to secondary depth ratios, and because all the computed T2s are correct, they will be band independent and the resulting relations T2 = f (T1) will just repeat that found from any one curve.

With standard light curves convertible to absolute units, the situation changes. Only one of the infinite pairs (T1; T2) satisfying (5.1.17) will also reproduce the absolute system flux for a given distance D. As we do not know D, we add the two constraints

and two passbands. The three equations (5.1.17, 5.1.18 and 5.1.19) are needed and sufficient to determine all three unknown quantities T1; T2, and D as stated above in the temperature-distance theorem. As a consequence, one standard light curve suffices to find (T1; T2) if D is independently known.

One has to keep in mind that this is the ideal situation. If the absolute flux calibration is not accurate enough or the observed data are too noisy, the temperatures and distance can appear larger or smaller than they really are, leading to an error in the temperature perhaps by several hundred degrees. Thus, it may well continue to be the case that light curves in more passbands improve precision and that spectro-scopic temperature determinations prove to be more accurate.

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