Reply To: CG Dra: a VSS campaign

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Paul Leyland

Eclipses No(s): 27, 28
State: Quiescent
Data Quality: Good (check star sigma = 0.045)

Holy cow. What’s going on here? I thought there was something wrong with my telescope.

How do you measure the bright spot here? I assume the quiescence is somewhere in the middle of the chart, at 16.89 mag, and the bright spot before the second eclipse pushes it all the way up to 16.75 mag – an amplitude of 0.14 mag. Note two-step increase in brightness in the orbital hump – first to 16.85 mag, then the second one to 16.89 mag.

The profile is a new type, U/H/A, never seen before. It is similar to U/L/S, except that here we see a very prominent bright spot.

Lots of flickering – at the bright spot apperance, orbital hump, at the minimum, and the egress. Note the post-egress 0.05 mag dip, then sudden rise of 0.1 mag, then ~ 0.5 mag dip. Flickering? I don’t think so. Exactly the same structure is visible in both eclipses tonight. Is this the second bright spot or, perhaps, stream overflow? Could the second bright spot/stream explain the 2-step rise, and the post-egress hump?

LOTS of questions.


I have been following this thread with interest and the quoted post, expressing a degree of incomprehension over the star’s behaviour, prompted me to contact an old friend, Phil Charles. Phil is a notionally retired but still very active professional astronomer who has specialized in the study of CVs of various types. He has given me permission to pass on his response.

Have to confess I’m not familiar with CG Dra, but that’s a very good light-curve for an almost 17th mag star (as expected for a 0.4m telescope in presumably a good site). It’s an eclipsing U Gem-type dwarf nova, here in quiescence, but I just looked at the AAVSO long-term light-curve and you can see that it outbursts quite frequently. That means it will be undergoing continuous mass transfer from the donor into the disc, and that displays itself as enhanced light when viewing either side of eclipse by the donor. In fact the hump here is quite broad, so it’s definitely an active system, and it’s clearly growing the disc in preparation for its next outburst.

Given the obvious data noise that is also visible in the comparison star, my reaction is that there’s nothing that unusual here. Let me know if you think I’ve missed anything.

  • This reply was modified 1 month, 2 weeks ago by Paul Leyland. Reason: Fix typpo