This week we have another look at the light curve of the long period variable V1376Cyg. This is a background giant star (Gaia distance about 4100kpc – or 13000 light years) and not associated with the foreground population of stars we are monitoring. The star is actually located in the overlap region between two of our target fields – PCyg and Berkeley86. Thus, it appears in large field of view data for both targets, basically doubling up on the data for this and other stars in the overlap region.

The estimated period for the object is about 390 days. In the figure we show the I-Band light-curve. It changes it brightness in this filter typically by about 3.5mag, so is probably some kind of Mira variable. We don’t show the light curves for the other filters, as our data actually does not cover the fainter parts of them. The reason is the very red colour of the star. The V-I colour is about 5mag when the star is bright, and probably a bit higher during the faint state. Thus, it will have a minimum brightness of about V=19mag or slightly fainter, and this is at least one magnitude fainter than the typical V-Band detection limit in our data.

This is a nice illustration how difficult it actually is to monitor stars like this in multiple filters to study their colour and brightness variations. If one wanted to do this for this star for all filters B, V, R, I then the I-band images need to be exposed so that at it’s brightest the star does not saturate (about I=10mag) and the B-Band images need to be deep enough to detect the star reliably at it’s minimum, i.e. at B=22mag. This is actually very difficult to do with a single telescope, as it requires to cover a range of 12mag, or very roughly a factor of 60000 in flux.