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\IBVShead{4614}{23 July 1998}

\IBVStitle{AAVSO Photoelectric Photometry of RU C\lowercase{am}}

\IBVSauth{J.R. PERCY, Y. TANG}

\IBVSinsto{Department of Astronomy, and Erindale Campus, University of
Toronto, Toronto, Canada M5S 1A7;\\ e-mail: jpercy@erin.utoronto.ca}

\SIMBADobj{RU Cam}
\IBVStyp{ CW }
\IBVSkey{photometry}
\IBVSabs{We report AAVSO photoelectric photometry of RU Cam from 1987 to 1997.}
\IBVSabs{The average amplitude is 0.2, with some fluctuations.  The period is}
\IBVSabs{22.20 days, as it has been for many decades.}

\begintext

RU Cam (HD 56167) is a population II Cepheid which, in 1965-66,
decreased its amplitude abruptly and drastically -- from one magnitude
to less than 0.1 (Demers \& Fernie 1966).  The amplitude fluctuated
around 0.2 magnitude from 1966 to 1982; the cycle-length period varied
between 17.4 and 26.6 days (Szeidl et al.\ 1992).  Some UBVR
observations have been published by Berdnikov \& Voziakova (1995);
although they state that the amplitude of the light curve is 0.03
magnitude, their Figure 1 shows it to be about 0.2 magnitude.  The {\it
HIPPARCOS} satellite monitored RU Cam between 1989.85 and 1993.21; the
star was classified as a periodic variable, with a period of 22.24 days
(ESA 1997).  The American Association of Variable Star Observers
has monitored RU Cam photoelectrically from 1987 to 1997.  The results
are described here.

A total of 109 differential V observations were made between JD 2447066
and 2450709.  The comparison stars were HD 56323 (F5) and HD 57201
(F8).  The V magnitude of HD 56323 has been well established, by other
observers, to be 9.05.  Observations were corrected for differential
extinction, and transformed to the V system using the catalog values of
(B$-$V), and transformation coefficients determined for each observer
from observations of blue-red pairs.  The standard error of the check
star observations was 0.012 to 0.03, depending on the observer.

The power spectrum of the observations was determined using the VSTAR
software developed by the AAVSO (www.aavso.org) as part of its {\it
Hands-On Astrophysics} education project.

The range of the observations was 8.34 to 8.68.  The highest peak in
the power spectrum of the 1987-1997 data was at a period of 22.20 days,
in excellent agreement with the period derived from the {\it HIPPARCOS}
photometry, and with the period (22.16 days) before 1966.  The phase
diagram for that data and period showed an amplitude of about 0.20 mag,
with relatively little scatter.

The 1997-98 data (Figure 1) is rather different; it is more typical of
a double-mode pulsator, in the sense that it appears to show systematic
variations in amplitude.  The data of Szeidl et al.\ (1992) occasionally
show a similar appearance.  The various forms of non-periodicity in RU
Cam may arise from cycle-to-cycle changes in either the phase, the
amplitude, or the shape of the light curve -- or even from the presence
of multiple periods.

\vfil\eject

\IBVSfig{8cm}{4614-f1.eps}{
The AAVSO photoelectric V light curve of the peculiar
Population II Cepheid RU Cam in 1997-98, relative to HD 56323 (V=9.05)}

Szeidl et al.\ (1992) suggested that the period of RU Cam fluctuates
over a wide range.  An alternative interpretation is that there are
random, cycle-to-cycle fluctuations in period, as in the Mira and RV
Tauri stars (Percy et  al.\ 1997); this possibility was also alluded to
by Szeidl et al.\ (1992).  A cursory application of the Eddington \&
Plakidis (1929) method for identifying such changes supports this
proposition.  The exact period found over any limited range of time
would then be affected by these random changes.

The low scatter in the phase curve from 1987 to 1997 shows that the
pulsation is now relatively stable, so that the 1965 decrease in
amplitude was an abrupt decrease from one discrete state to another,
with no significant change in period; the more complex 1997-98 data
remind us that this star is still not predictable, and is well worthy
of study.

{\bf Acknowledgements.}  We thank the many AAVSO photoelectric
observers for their contribution to this project, and especially the
archivist Howard J. Landis.  One of us (YT) was a participant in the
University of Toronto Mentorship Program, which enables outstanding
senior high school students to participate in research projects at the
university.


\references

Berdnikov, L.N., Voziakova, O.V. 1995, IBVS, No.\,4154.

Demers, S., Fernie, J.D., 1966, {\it ApJ}, 144, 440 

Eddington, A.S., Plakidis, S., 1929, {\it MNRAS}, 90, 65

ESA, 1997, {\it The Hipparcos and Tycho Catalogues},
ESA SP-1200

Percy, J.R., Bezuhly, M., Milanowski, M., Zsoldos, E., 1997, {\it
PASP}, 109, 264

Szeidl, B., Ol\'{a}h, K., Szabados, L., Barlai, K., Patk\'{o}s, L.,
1992, {\it Comm.\ Konkoly Obs.}, 11,~247
\IBVSlink{http://www.konkoly.hu/Mitteilungen/97.html}

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