[Aavso-photometry] B-V from V-I

[Arne Henden]

Michael Koppelman wrote:
> I have a sequence where I have V, V-R, V-I and R-I. Is there a standard 
> way to compute what B-V would be? The sequence has several hundreds 
> stars in it so I need something I can do programmatically.
> 
Sure; this is a common situation.  The best reference is available through ADS:
Caldwell, J. A. R.; Cousins, A. W. J.; Ahlers, C. C.; van Wamelen, P.;
Maritz, E. J.,
Statistical Relations between the Photometric Colours of Common Types of Stars
in the UBV(RI)c, JHK, and uvby Systems
S.A.A.O. Circulars, Vol. 15, p. 1, 1993

For dwarfs, the relation is approximately
(B-V) = -0.07 + 0.88*(V-I)

In other words, (B-V) is usually slightly smaller than (V-I).
Arne