c1 Hbeta -> BC, Teff, log(grav)
Balona 1984, MNRAS 211, 973
Calibration of effective temperature, bolometric correction and 
mass for early-type stars using Stroemgren photometric indices
Needs: c1 Hbeta

Valid for:
-0.20 < c0 < 1.00
beta < +0.304*c0 + 2.522
beta > +0.304*c0 + 2.722

c0     = c1 - 0.19*E(b-y)
[c]    = log( c0 + 0.200 )
[beta] = log( beta - 2.500 )

BC = +0.2900 + 2.8467*[c] + 2.8334*[beta] + 0.6481*([c]^2) -
             - 0.2997*[c]*[beta] + 2.1487*([beta]^2)

standard deviation = 0.11mag

log Teff = +3.9036 - 0.4816*[c] - 0.5290*[beta] - 0.1260*([c]^2) +
                   + 0.0924*[c]*[beta] - 0.4013*([beta]^2)

log g = +6.1788 - 4.1089*[c] + 5.0601*[beta] - 0.6332*([c]^2) -
                - 1.2759*[c]*[beta] + 0.7562*([beta]^2)

standard deviation in log Teff = 0.013