Sat, 01/11/2020 - 14:58

As you know, NGC 7790 is one of the standard transformation fields.  One of the reasons I started observing this field many decades ago is because there are a number of variable stars in or near the cluster.  My three favorite ones were the three cepheid variables: CEa, CEb and CF Cas.  CEa/b are a neat pair, separated by only 2.7 arcsec.  All three have 5-day periods, highlighting their common origin.  If you observe this cluster, you should submit data on CF Cas and keep its light curve current.  CEab is a nice pair to use for determining seeing, but I don't recommend submitting data on either star unless you have excellent seeing and can split the pair cleanly.  I do have a nice set of CEab photometry, where I've psf-fit the stars, in case someone wants to write a paper about these stars.

However, there is another neat variable only a few arcmin SE of the cluster center: the tenth-magnitude QX Cas.  This is a possible triple system, with the inner pair an eccentric eclipsing B1V binary with 6.0047d period, and a third star that precesses the line of nodes.  This means that it was an eclipsing system in ~1950-1980, but since then has NOT been eclipsing. Kjurkchiva et al. (2007) presented a spectroscopic orbit with e=0.22.

QX Cas has the rare distinction of being a star that has quit varying.  Moffett and Barnes (1983) indicated that the amplitude of the eclipse was less than Sandage found a couple of decades earlier, and soon thereafter, observers reported no variation at all.  Bonaro et al. (2009) presented a paper at an AAS meeting that highlighted this.  I know of no recent paper, and the few data points in the AID do not show any variability.

I've been observing QX Cas off and on for decades, waiting for it to resume its eclipses.  I was thinking that it would be a neat project for the CCD observers to obtain as complete a light curve as possible before it vanishes this season, and then build a similar light curve once per year for a while to see what happens.  As an incentive, you get to observe NGC7790 and obtain your transformation coefficients.  As a further incentive, I'll be happy to inspect the photometry submitted to the AAVSO/AID and give suggestions for improving your observational techniques (hence the posting to the Photometry Forum).  As with the campaign for BL Boo, you will have to suffer public comments, as I want everyone to learn from these observing campaigns.

QX Cas is a nice early evening object for the next month or two for northern observers.  Submit some data!  Especially those of you not in North America should contribute, as the 6.00day period will make obtaining a complete orbital coverage difficult!  If you have observations of NGC7790 taken over the last few years for determining your transformation coefficients, you should inspect them to see if QX Cas is visible, and submit its measures as long as it is not saturated.


American Association of Variable Star Observers (AAVSO)
QX Cas

Hi Arne,

I have BVI observations I took in 11.07.19  of NGC7790 with  QX Cas in the FOV.

I will share them with you in the VPHOT Image list.

Erik (SDM)

Royal Astronomical Society of New Zealand, Variable Star Section (RASNZ-VSS)
QX Cas

Hi Arne,


Good to see a post about real observations rather than these interminable discussions of theoretical relationships which probably mean nothing anyway.


That's a very interesting group of Cepheids, particularly when they're so close together and similar in behaviour.  At Auckland we monitored the bright Cepheids in Crux for many years - I can't remember whether we found anything of interest such as period changes - but I've changed my interest to the longer period ones >20 days, some of which do show such period changes, also period alternations in some.


The closeness of the A and B pair brought back memories of KZ Pavonis, an EB we measured for Ed Budding.  This had a companion star of similar magnitude at a separation of 5 to 6 arcseconds and in those days of fixed focal plane aperatures we included both in the 31" field.  On good nights - and at the Auckland Observatory in those days they were very good - we'd measure these individually at the edge of the aperture during non-eclipse.


Adding the signals together they came to around 10% more than the jointly measured intensities.  So we deduced individual values which were exceedingly similar to the values derived by Ed's third-light excluding software.  But there were no other stars to get in the way and at 7.71 to 9.30 it's a little brighter than your Cassiopeia triplet.


Best wishes for 2020.  I haven't done any observing for a long time but the paper on QZ Carinae is in the hands of the AAVSO editorial team now and I will tidy up V777 when the weather cools a little so I can think.