# What does the Reddening line represents in this U-B vs B-V plot?

The plot is from this paper (pg-8).

They have also talked(in the same paper) about how central stars from galactic longitude range $$20^\circ-80^\circ$$ will have a very unusual Reddening line (pg-9). The current plot is for central stars from the galactic longitude range $$200^\circ-300^\circ$$

The group of planetary nuclei plotted here are all those occurring in galactic longitudes 200o -300°. In the longitude range from 20° to 80° the points fall to the right of the reddening line in Figure 3 (not plotted). If the latter energy-curves are well-behaved, a very unusual reddening line would be needed to explain their position in the diagram. This is a puzzling result which is unexplained at present.

My interpretation of Reddening line: It is showing that if we plot real star data where the sample contains all the stars from bluer end to redder end on this U-B vs B-V plot, we will get the Reddening line on the plot.

Is this interpretation correct?

i.e. It is an equation on the colour-colour plot of the form (for this colour-colour plot) $$E(U-B) = f(E(B-V)) + c\ ,$$ where $$E(U-B)$$ is the colour excess in $$U-B$$ due to interstellar reddening, $$E(B-V)$$ is the colour excess in $$B-V$$ and $$c$$ is a constant chosen so that the locus would go through a star of the same spectral type but with no interstellar reddening.
Typically, for low reddening and a typical interstellar medium $$E(U-B) = 0.72 E(B-V) + 0.05 (E(B-V))^2\ ,$$ which I imagine is the line drawn on the plot you show.
To establish the reddening to a particular star you can follow it back from its observed position in the colour-colour plot, along a reddening line, until it intersects the main sequence locus (if it is a main sequence star). The intersection would give its unreddened, intrinsic colours and the x-axis increment through which you traced it back would give its $$E(B-V)$$.