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Z Tau Reminder

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ocn's picture
Z Tau Reminder

Hi All,

Caution when observing the (period change) mira Z Tau as it fades to minimum. The very close 13th mag companion is apparently frequently mistaken for the mira itself. Currently Z Tau is 15th mag and fading.

kind regards,




hhu's picture
Z Tau

  already 15 years ago I mentioned this on the AAVSO mailinglist.  And still some observers seems to estimate the close companion.  It's even mentioned on the chart. Even the CCD observers are wrong. Don't anybody read the comments on the charts?

ocn's picture
Z Tau Reminder

I feel your pain. A look at the note regarding correct identification/companion (on the VSX page for this star) would be a tip off as well. Z Tau is possibly one of the more unusual stars of its' class. Pity not more care is taken. The pretty much unique FF variable FG Sge is kind of in the same predicament when faint. In the latter case, not only is there a closeby 12th mag mimic that sometimes fools observers, but an even closer 17th mag component ready to do the same to CCD folks. Its all documented in VSX.



hhu's picture

I think when someone should look deeper in some variables they would found similar cases.

lmk's picture
Not some, almost all!

Unfortunately, the 5" bright companion seems too much for most CCD, and all visual observers to handle! As the attached LCG shows, over the past 3 years or so, even though many more "new" CCD observers have started on this star, they are almost all combining the stars.

There are only a very few isolated "good" faint observations, and so few, its even hard to say what the true minimum of this star is! It appears that a large observatory-class scope and CCD will be necessary to truly resolve and determine the minimum when it is near or below mag 17, with such a bright neighbor.

I can try visually with my 41cm at extremely high power and good seeing, to determine if it is feasible for me.



File upload: 
HQA's picture
Z Tau's companion

Hi Mike,

Z Tau was the subject of a JAAVSO paper by Walt Cooney et al.:


where the UBVRI magnitudes of the companion are listed.  Because of this, you can use the Nemesis spreadsheet by Lew Cook to separate the variable's magnitude when both stars are measured simultaneously.  The minimum during this period was about 18th, pretty faint to split even with PSF fitting.

There are many variables with close companions that cause photometric problems, especially when the variables are in the Galactic plane.  These are difficult targets, ones that the professional surveys often cannot measure.  A diligent amateur can make important contributions in such cases!  Mike Simonsen created a list and a web page of typical LPV examples:


lmk's picture
Increased errors

Hi Arne, I was thinking of what happens with this common method, of subtracting the contribution of the bright component from the total flux, to get the faint one. The measurement error reported for the combined would be significantly larger for the faint component, because when you subtract the adu of the bright component from the total, the resulting adu is a small number, but still subject to the error of the combined measurement! 

This factor looks like it would be (10^(0.4*[mlimit-mbright]) + 10^(0.4*[mlimit-mfaint])) / 10^(0.4*[mlimit-mfaint]). So, for the case of an 18th mag variable with 13.5 mag companion, with instrumental limit of 19 mag, and say 0.02 combined mag error, this would be a factor of (10^2.2 + 10^0.4) / 10^0.4 = 64 times worse error for the faint one. That's equivalent to a 1.28 magnitude error with this method, on the faint component, vs. a direct measurement of the faint component without a contaminating neighbor., due to SNR alone

So, this may explain some of the wide variations of reported minimum of Z Tau, in addition to the random statisitical component of having very few faint measures.


Andrew Pearce
Andrew Pearce's picture
Z Tau discussion

Hi All

This is a very timely discussion as Z Tau will be the February LPV of the month! This was unscripted!

The article will hopefully alert more observers to the pitfalls of observing Miras like Z Tau with close companions.



Sebastian Otero
Sebastian Otero's picture
Variable stars with companions

It is certainly a good time to update our Variable Stars with Companions page.

We have added a 2018 version of a spreadsheet including all stars with chart notes calling the observers' attention to the presence of close companions.

And as many of you have said, there are probably more objects that have a companion and we are not aware of. We have added hundreds of chart notes this year. You should pay attention to them.

And take the chance to download the new version of our spreadsheet and know about HQ policy for the observation of these objects:

You can then sort the objects as you wish. E.g.: you can see how many miras are known to have close companions if you order them by variability type.


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