(cba:news) comments re HP Lib... and AM CVn

[Joe Patterson]

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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