This week we look at the object V1319Cyg, which is situated in the field around the Berkley 86 open cluster. We show the light curve in V and I in the above plot. For clarity we zoomed into two years of data, taken in 2020 and 2021. A one can see, this field is one of the lesser observed targets in our list. However, the variability of the object can clearly been identified. It regularly varies by about one magnitude. So what kind of object is it?

The SIMBAD entry classifies it as a Mira variable. This seems obviously wrong. Miras are pulsating  stars that have periods above 100 days and amplitudes larger than 1mag. The period of this object is 41.4d and the amplitude just below one magnitude.

The ASAS-SN survey has classified the object as a semi-regular variable. This usually implies periodic variations with a changing amplitude. While the light curve looks like it could have a changing amplitude, this is caused in part by the observing cadence – we do not have data for every maximum and minimum, or the fact that esp. in the I-band, the object is close to saturation during the bright state. The phase folded plot of the light-curve, even in the ASAS-SN data, clearly shows a completely periodic light curve. So it is not clear where the classification comes from.

The phase folded data looks very much like a bona-fide Delta-Cepheid variable. So could this be? Let’s check. We can look up the parallax from Gaia and work out a distance of about 4400pc. With the known apparent magnitude of G=10.06mag, this converts to an absolute magnitude of -3.15mag. Probably the object is brighter than that by at least one or two magnitudes, as we have not accounted for line of sight extinction. So about -4mag or maybe even -5mag or above. This is in line with the period luminosity relationship for both, classical and TypeII Cepheids.

And indeed: One of the auxiliary catalogues from the latest Gaia data release classifies the object as Delta Cepheid star … should have looked there first 🙄