[vsnet-rcb 15] re vsnet-rcb 13 & U Cyg

[no name <crawl@zoom.co.uk>]

Dear RCB devotees

I note from vsnet-rcb 13 that there a few of you out there who are
experts on the lightcurves of stars denoted as being of type RCB.

U Cygni is a Mira Carbon star.

Does anyone know of either i) an RCB star that still manages to show
periodic pulsation of undiminished amplitude during obscuration events,
or ii) an eclipsing Mira?

[I know some RCB stars are _supposed_ to show evidence of Cepheid like
short period pulsation].

When you append BAAVSS & AFOEV data for U Cyg from the 1920's to date,
[that's _append_, a lot of Miras were dropped from the BAAVSS programme
in 1970] you find a lightcurve for this Mira that underwent an
"eclipsing" event over 20 years starting around 1954/5.

I analysed the living daylights out of this lightcurve a year or so ago,
and the way the differing periodicities fitted together, in tandem with
the overall shape and evolution of the lightcurve at the time, suggested
to me that it must have a smaller star as companion that eclipsed it.

Basically a long _clean_ and moderately shallow parabolic dip in mean
magnitude occured throughout this time, whilst at the same time the
_amplitude_ and _pulsation period_ remained more or less unaffected with
respect to either before, or after the event [over and above the usual
Mira shennanigans of small stochastic variations in mean period, and
variant extreme amplitudes].

I cannot see an obscuration event of the RCB type leaving amplitude
intact whilst depressing mean magnitude.  Similarly, symmetry of max and
min distance from mean magnitude remained more or less the same.

Intensive fourier decomposition techniques reveal the periodicity of the
"eclipse" to only be extant in the data during the actual event itself
[ie it was not a modulation effect], and independent of the pulsation
period[s].

I personally think it is the only known case of an eclipsing Mira, but
carbon cloud possibilities remain due to it's nature. 20 years is a bit
long, mind. Potential affects purely due to the nature of the dataset
itself have been dismissed to my own satisfaction, though some are bound
to question that aspect.

So, in summary, how many eclipsing Miras are known, if any? Do RCBs
known to pulsate at short period level reduce in apparent pulsational
amplitude during obscuration events [though the pulsations are of such
low amplitude in the first place that I suppose it wouldn't take much
for them to dip below detection level]?

[A potential problem with the data I've recently noted is that an
optical comes at 65" is catalogued as NSV 13020, which could be a
problem if it is used a comparison star.  However, then the discussion
would be about how _this_ G0 star could have a smooth dip of up to a few
tenths of a magnitude over 20 years!!!  Besides, I've examined data
resulting from iffy sequences before, and although certain sequence
stars lend themselves to use at certain magnitudes, the U Cyg lightcurve
is too smooth for such effects, unless we have the monumental
coincidence of absolutely *everybody* _always_ using this comparator
*throughout* U Cyg's mag 6 to 12 range!!!]

Cheers

John

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