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Archival light curves from the Bamberg Sky Patrol: CF Octantis, 1964-76
We use the archive of the Bamberg Sky Patrol to obtain light curves ofthe active K subgiant CF Octantis (HD 196818) for the interval 1964-76.Digitized images of the field near CF Oct were obtained with a flat-bedscanner. Aperture photometry was performed of photo-positives of theseimages. Using a transformation to second order in plate magnitude, andfirst order in B-V, for nine field stars for each plate, the Bmagnitudes of CF Oct were obtained for just over 350 plates. Theestimated precision of an individual determination of the B magnitude ofCF Oct is 0.05 mag. Analysis of the resulting data reveals the known 20d rotational variation of this star, and shows the evolution of thelight curves from year to year. We obtain light curves with good phasecoverage for 1964 to 1969 inclusive, partial light curves for 1970 and1976, and a few data points from 1971. The amplitude of variation rangesfrom ~0.2 to ~0.4 mag. There is evidence that the characteristicrotation period of the star in the 1960s was slightly less than thatmeasured from photoelectric photometry in the 1980s.
| An Historical Light Curve of CF Octantis from Digitised Images of the Bamberg Plate Archive
Digital images were made of Bamberg Observatory Sky Patrol plates of thefield surrounding the active-chromosphere star CF Octantis (HD 196818).These images, taken with an inexpensive camera, were analysed usingstandard aperture photometry techniques. Good agreement was found withcatalogued photographic magnitudes for stars in the range mpg~8.5 to ~10.5. The root-mean-square deviations in themeasured differences for non-variable field stars was found to be oforder 0.10mag, although a small number of larger differences occurred.For CF Oct, a period search of data from 1966 recovered the known 20dvariation due to starspot rotational modulation, with a range ofvariation of order 0.5mag photographic. For active-chromosphere starswith moderately large photometric variations (>~0.3mag), and moderateto long rotation periods (>~1 week), careful analysis of similarlyobtained digital copies of archive plates may provide valuable insightsinto historical actvity.
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