MBCAA Observatory

VY Cet: a contact binary in a ternary system

Observed: 26 Dec 2006, 3 Nov 2007, 30 Jan 2009, 31 Oct 2014

Michel Bonnardeau
23 Nov 2015


Time series of this contact binary in a ternary system are presented. 3 ToMs are measured along with the B-V and V-Rc colors.


VY Cet is a contact binary with a period P = 8.18 hr. According to Liakos & Niarkos (2008) (hereafter L&N), this period is increasing on a time scale of P/(2*P')=4.0 Myr, due to mass transfer. They give the ephemeris t(n) = T+P*n+b*n2 with
T = 2,440,282.14835(132) HJD (1969 03 01)
P = 0.340,809,1(1) day
b = 0.197,20(1)*10-10 day.
Furthermore L&N report that the binary is orbiting a third body with a period of 7.07(6) yr. See also Kozlowski et al (2011) and references therein.


The observations were carried out with a 203mm SC telescope, Johnson B and V filters and a Cousins Rc in a filter wheel, and a SBIG ST7E camera (KAF401E CCD). The exposure durations were 200s whatever the filter. The following table gives the numbers of useful images:

Season Nb of images
B V Rc
2006 16 17 0
2007 0 46 48
2009 9 12 12
2014 23 23 23

For the 2006, 2009, 2014 observations, the comparison star is 14CMC 015003.8-193156 (GSC 5857-01140) with the magnitudes B=12.702 V=12.120 Rc=11.790, computed from the CMC14 r' magnitude and the 2MASS magnitudes, owing to the transformations of Bilir et al (2008) and of Smith et al (2002). This a F5-G0 star.

For the 2007 observations, the previous comparison star is not in the field of view. Therefore, another comparison star is used: 14CMC 014852.0-193724 (GSC 5847-02044) with the magnitudes B=11.525 V=10.985 Rc=10.678, computed the same way as with the previous comparison star. This is also a F5-G0 star. However, for the 2007 observations to match the other observations, one needs to correct them by adding to the V magnitudes -0.06 and to the Rc magnitudes -0.02.

An example of light curves:

Blue: magnitudes with the B filter, Green: with the V filter, Red: with the Rc filter. The error bars are the 1-sigma statistical uncertainties (computed as the q-sum of the uncertainties on the variable and of the comparison).

The check star is GSC 5857-01297. The measured magnitudes and the uncertainties on its measurements are given in the table below. The first ± is the average of the statistical uncertainties (computed as the q-sum of the uncertainties on the variable and of the comparison), the second one is the standard deviation:

Season Magnitudes
2006 12.962±0.024±0.024 11.794±0.009±0.013 none
different comp
none 11.857±0.009±0.008 11.219±0.011±0.014
2009 12.934±0.027±0.030 11.782±0.010±0.014 11.166±0.014±0.020
2014 12.933±0.030±0.032 11.763±0.012±0.016 11.153±0.017±0.024

The observed times of minimum

In 2006, a time of maximum (not minimum) is observed at 96.283 THJD. This gives a time of minimum by subtracting P/4. One has then the following times of minimum (the cycle numbers are computed from the L&N ephemeris):

Season HJD-2,454,000 cycle nb
2006 96.198 ± 0.005 40,533
2007 408.388 ± 0.0015 41,449
2009 none
2014 2962.423 ± 0.0015 48,943

According to the ephemeris, these are all primary eclipses.

The corresponding O-C diagram from ephemeris of L&P:

Phase plot

With corrections to take into account the period variations, the phase plot of all the data is:

Blue: with the B filter, Green: with the V filter, Red: with the Rc filter.

After interpolations (because the magnitudes with different filters are measured alternatively, not simultaneously), the B-V and the V-Rc phase plots are:

B-V=0.74, V-Rc=0.39.

As usual with contact binaries, the color does not vary with the phase, i.e. the two stars are at the same temperature.


Bilir S., Ak S., Karaali S., Cabrera-Lavers A., Chonis T.S., Gaskell C.M. (2008) MNRAS 384 1178.

Kozlowski S.K., Konacki M., Sybilski P. (2011) MNRAS 416 2020.

Liakos A., Niarkos P. (2008) IAU Symposium, arXiv:1404.3124v1 [astro-ph.SR].

Smith J.A., Tucker D.L., Kent S. et al (2002) AJ 123 2121.

Technical notes

Telescope and camera configuration.

Computer and software configuration.

Data processing.

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