Mon, 09/21/2020 - 14:20
V Del is a Mira with a published spectral class of M4e. It has just passed through minimum. It typically shows color indices pretty typical for an LPV: (V-R) of +4.0, (R-I) of +3.0. However, on the current minimum, I've seen (B-V) go negative, while maintaining the strong red color in its other indices (B-V = -0.19, V-R = +4.17, R-I = +3.16). At first, I discounted these unusual B magnitudes due to the pretty poor SNR at the bottom of the lightcurve, but I'm realizing it isn't just one spurious data point.
Is there a credible physical explanation for one of these red stars to be unusually blue? I seem to remember that some of the H emission lines are located in the blue wavelengths. Can these intensify near minimum?
What filter are you using? Some B interference/dielectric filters have a red leak, which results in an incorrect magnitude estimate for very red stars. A Mira variable does not have a negative (B-V) index.
I'm using an Astrodon Johnson B filter. My B-V indices are usually between +1.5 and +2.0 for Miras, although last night I saw a B-V of +3.8 for V Aur (which I've seen go to as much as +6 when V Aur is near minimum).