[Aavso-photometry] Transforming

[arne]

Barbara G. Harris wrote:
> Here are the TCs that I obtained about 1 month ago.  I re-imaged M67 a few
> nights ago to see how repeatable my numbers were but I have not performed
> the calculations yet for those images:
> Tbv:  1.268
> Tvr:  0.981
> Tri:  0.868
> Tv:  -0.189
> Tr:  -0.173
> 
> What do the negative numbers for Tv and Tr signify?  How do I know if a
> particular star that I am observing has a constant color?
> 
To answer your question regarding negative numbers, I need to know
which color index you were using for each coefficient determination
(for example, Tv could be calculated using (B-V) or (V-R) or (V-I)).

Usually low amplitude means small color change, which can be neglected
to first order.  CVs tend to have constant color during outburst.
Other stars, like pulsators, change color during their pulsational
cycle.  So *that* answer is: it depends on the variability type.
You can determine if a star has color change yourself if you use more
than one filter, since what you are looking for is a change in color
in comparison to a nearby comparison star, which can be calculated
differentially (you don't need absolute values, just knowledge of
whether it is different today than yesterday).
Arne