V538 Cas

BD 60deg 201 is a star in Cassiopeia. Its variability was discovered by R. Weber. It was one of 75 new variables announced in 1958 after analysis of 2 068 photographic plates exposed between 1942 and 1958 using three cameras with apertures of 46, 71 and 83 mm. The star was found to vary between 9.0 and 9.6 photographic magnitude. Weber, who did not publish a light curve, suspected it could be an eclipsing variable of the Algol type.1

The next step was when Klaus Häussler used 135 sky patrol plates to study the star. Häussler was certain: the star “is coloured. Variations are irregular between 9m.44 and 10m.01 ph. The star is an Isb-typ variable”, he wrote in a 1974 paper.2 That the star had colour was confirmed by spectral class data: it had been classified as a K5 star.3 The next year, in 1975, the international clearinghouse for variable star data, Kukarkin’s group, officially designated BD 60deg 201 a variable; they called it V538 Cas.4

Case closed, one might think. A rapid semi-regular variable star with fairly large amplitude.

But now things get a bit more problematic. Variability on the order of 0.6 mag seemed to be ruled out once we got into the era of photoelectric photometry. A study by Henry et al found slight variations in the star but could not confirm variability of large or moderate amplitude in V538 Cas. Also, they found that the star is not a K5 star after all but an M0IIIb.5 Data from the Hipparcos mission seems to show some variability but on a very modest scale on the order of 0.03 mag; it is classified as a type U Hipparcos variable: “unsolved variable which does not fall in the other categories”.

Data from the leading variable star organisations is not of much help. Observations from the AFOEV archive or the AAVSO archive is not very clarifying; the star has not attracted many observers, and the data is not easily interpreted. It doesn’t show up in NSVS (I think), but that could be because it is too bright for that survey. The All Sky Automated Survey has not yet released and/or analysed data for Cassiopeia, and even when that data is released, there is a risk that we might not get any wiser regarding V538 Cas, since the star is close to the saturation limit (8 mag in V) of that survey.

What is V538 Cas?

Is it an example of the evolution of astronomical measurements? A variable that seemed to be fluctuating around 0.5 mag on small-scale sky patrol camera plates is suddenly reduced to variability around 0.01 mag once more precise photoelectric methods are used?

Or is it an example of something unknown lurking in the heart of Cassopeia, something that shows up only now and then, with significant activity but then keeps still for years on end? Remember, Weber thought it could be an Algol variable. The 200+ measurements made by Hipparcos and the Henry et al team could very well have missed something, for example an eclipse. Perhaps further clues could be found through reanalysis of the patrol camera plates used by Häussler; they are still available.

My guess is the first alternative. But who knows? Not many people observe V538 Cas these days; there’s a slight, slight chance that we might be missing something …

  1. R. Weber, “Catalogue d’étoiles variables nouvelles,” Journal des Observateurs 41 (January 1, 1958): 74. []
  2. K. Häussler, “Observations of 6 Csv-Stars on Sky Patrol Plates,” Information Bulletin on Variable Stars 887 (Maj 1974): 2. []
  3. Denise de Smet-de Potter and M. Neyts, “Étude spectrophotométrique et statistique d’une région obscure dans Cassiopée,” Annales d’Astrophysique 16 (January 1, 1953): 1. []
  4. B. V. Kukarkin, P. N. Kholopov, and N. P. Kukarkina, “61st Name-List of Variable Stars,” Information Bulletin on Variable Stars 1068 (November 1, 1975): 1. []
  5. Gregory W. Henry et al., “Photometric Variability in a Sample of 187 G and K Giants,” The Astrophysical Journal Supplement Series 130 (September 2000): 201-225. []
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