Abstract: Dr. Tom Macarrone requests photometry of a V=7.8 eclipsing binary with a 2.08 day period during the eclipse, in several bands. B, V and R are the most useful. The durations of the eclipses are about 8 hours each. In TESS, the eclipse depths are about 0.35 and 0.06 magnitudes for the primary and secondary eclipse, but we need several bands of data to solve the binary, and the very broad TESS band is not very well suited to this purpose. Relative photometry of the eclipse depths with half hour cadence and 1% accuracy should allow estimation of the temperatures and radii of both stars. The full eclipse need not be covered by a single observer in a single night; the solution can gradually be put together with data collected from many telescopes on many nights.
Justification: CP Gru is an approximate 7.8 magnitude eclipsing binary with an A star primary and strong X-ray and radio emission and repeated strong millimeter flaring with the South Pole Telescope. As X-ray and radio emission is generally associated with magnetically active stars, the companion to the A star must be responsible for the flaring. We wish to obtain photometry of the eclipse in several bands to obtain the temperature and radius of the companion star.
If anyone else has any questions about this object or campaign, please let me know. Once we have a decent amount of data, we can have a video call to discuss what we’ve learned here.
I am preparing to observe this binary pair. I have the advantage of being located in the southern hemisphere. This also means we currently have longer hours of darkness, since our winter solstice occurs on June 22, 2026. I usually work with a V (Johnson) filter and ensemble photometry. I also have a B (Johnson) filter, but to-date, I have not been able to calculate transformation coefficients. I hope the ensemble photometry in Vj will be sufficient.
Would it be useful to have simulteneous B,V,R values via the use of tri color imaging? I do that with the Red Dwarf group for EV Lac and AD Leo. The results are in line with others doing B and V individual filters. I will be able to run a full night’s cadence as well. Also, is transformed vs. untransformed data significant for you? I noticed that Josch is submitting observations so I can also coorrelate with his values for additional validation.
Yes, tricolor imaging would be fantastic. I think we can probably figure out how to work with untransformed data if need be, since we know the baseline colors for the star when it’s not eclipsed, but transformed would be better.
Based on the submission of data to the AAVSO database, it seems that CP Gru just came out of eclipse and the B-band dip is very large (about 1 mag) compared to V and R band. Of course so far too few observations have been submitted to the AAVSO.
Josch
I’ve taken a look at the phase-folded light curves, and they are looking terrific. We still need a bit more coverage, especially around the secondary eclipse. For the primary eclipse, we might need a bit more data, too, but we’re close to having good eclipse depths in multiple filters.
Thanks! The data look great. The period is well known, and it’s 2.08623 days. In think your data set just isn’t long enough yet to get reliable periods out, especially for something like this which is highly non-sinusoidal.
I’ve taken images over two nights, but I have a few questions before submitting them. I’m not very experienced, and I would like to know if I’ve done things correctly.
The star is very bright, so I’m taking very short exposures. Is this OK?
On the second night I forgot to use the V filter. Is it OK if I submit them? I can upload them as “Unfiltered with V zeropoint” (or “CV” for short).
Since I only have a V filter, I will be submitting untransformed data. I understand this is acceptable.
I have taken data for a couple of nights now (about 8-hours per night) and have run into an issue. I have to use an F-scale chart but there are no comp stars on it. Looking at the E-scale chart does show three of them but even offsetting CP Gru to close to the edge of my field does not make them visible in my images. I asked for a new sequence about a week ago but so far have not heard anything back from the sequence team. Any suggestions?
1: How short? Are you worried about scintillation mattering on that timescale?
2:As long as they are labelled correctly, there’s no harm in submitting them, and someone else might use them down the road, but it probably won’t help much for my project.
The exposure time I’m using is 3 seconds. With exoplanet transits, stars are dimmer, and I normally expose for 1 or 2 minutes. That is why 3 seconds feels too short for me.
I used 3 seconds to avoid the sensor reaching the linearity limit, and to keep the FWHM between 3 and 5 pixels.
My field of view is about 14 x 14 arc minutes. The published comp stars are just outside of it even if I offset. I have a few things that I am going to try but I thought that perhaps someone might already have a ready solution worked out for situations like this.
