Hi!
Yes, same air mass. They were identified as ref stars for SS Cyg, or rather HD207673 was and the others as supplement stars for checking precisely the instrumental response. They were observed at roughly the same air mass.
Very helpful indeed, with your analysis. May I ask: how do you determine the extinction and reddening? Very good to know.
Also: I came upon another hypothesis tonight: My profiles might be tainted by skyglow. I tend to take the ref star first, and in the bright summer nights here in Sweden, the sky is not fully dark (I have relied on ISIS being capable of handling that….). So it might be that the different profiles are captured at times when the skyglow was significantly different. I need to check this more. If it makes sense.
Magnus