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(cba:news) comments re HP Lib... and AM CVn (Joe Patterson) [2019-06-18T15:01:59Z]


Hi CBAers,

These stars - one celebrated, and the other deserving to be - are really unique among CVs. Their orbital periods are very short of course (17 and 18 minutes), they have dominant superhumps, and the spectra sre basically pure helium. But the property I find most remarkable, and never commented on, is the lack of variability in the light curve - apart from the orbital and superhump periods. The "flickering" never exceeds 0.02 mag, and night-to-night variations never exceed ~0.05 mag.

The underlying reason for such constancy is a puzzle, but not yet one with a convincing explanation. I thought I would append one long night of HP Lib light curve, courtesy of Gordon Myers, to illustrate what the raw light curve looks like. You can see the effect of atmospheric extinction (actually differential extinction, arising from the bluer color - and thus greater extinction - of HP Lib). To analyze the light curve(s) over weeks, as we do, we (I) have to correct for this extinction - although sometimes by simply *deleting* high-airmass observations, since data quality sometimes degrades quickly there.

Anyway, you can see the 0.05 mag superhumps, and the extinction. With a lot of data spread around the world, you can dig out the orbital signal as well, although it's ~7X weaker (and thus invisible, except in a power spectrum). Gordon and Berto have been carrying most of the water on this one, with LONG time series.

Great evening target in April-May-June.

joe

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