I have been looking at Color Contrasting double stars and comparing their color differences to their differences in their spectral types. When I plot these differences there is a reasonable correlation as seen below. One of my data points is the optical pair S639 in Virgo which is a clear outlier looking at my plot and I suspect the spectral class of the magnitude 10 secondary which is listed as K0 in Simbad is in error. Looking at my photos and its color index I suspect it is at least a late F or early G star. Sky Safari lists the spectra of the primary as M0 III which also makes the spectral difference a bit greater but this measurement was from 1926. Is there a chance that someone here with a spectrograph could take a look at the spectral class of these two stars?
Edited by ssmith, 06 June 2020 - 01:47 PM.