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\IBVShead{5367}{28 January 2003}

\IBVStitle{LX Cygni: a Mira Variable with a Drastic Period Increase}

\IBVSauth{Templeton, M. R.; Mattei, J. A.; Price, A.}

\IBVSinsto{American Association of Variable Star Observers (AAVSO), 25 Birch St., Cambridge, MA 02138, USA; email: aavso@aavso.org}

\SIMBADobj{LX Cyg}
\IBVStyp{ M }
\IBVSkey{Miras: photometry, Miras: asteroseismology}
\IBVSabs{The Mira variable LX Cygni has been undergoing significant period}
\IBVSabs{increase over the entire span of the AAVSO archives (JD 2439800 -}
\IBVSabs{2453000) from 460 d to 580 d.  The period change appears to be}
\IBVSabs{continuous rather than a discrete jump, which argues against a} 
\IBVSabs{mode switch as the cause.  LX Cygni may be in the midst of a}
\IBVSabs{thermal pulse, and more observations are encouraged.}


\begintext

LX Cygni -- $\alpha$:21\hr 55\mm 57\fsec03; $\delta$:+48\deg 20\arcm 52\farcs6 (J2000) -- is a 
poorly-studied Mira variable of spectral type S.  Although 
this object has been studied spectroscopically as part of several S-star 
surveys, little has been published about its pulsation behavior beyond its
discovery and subsequent description in the {\it General Catalog of Variable
Stars} (GCVS).
Variability was first noted by Hoffmeister (1930).  Additional observations
were made by Olivier et al.~(1940), and the period was determined to be about 
461 days, with photographic maxima and minima of 11.9 and 16.5 magnitudes,
respectively (Prager \& Shapley 1941).  
Semakin (1955) summarized the work to date on
LX Cyg, noting that the period had been variously measured between 454 and
465 days.  Finally, the GCVS 4th edition (Kholopov et al. 1985) lists the 
period as 465.3 days (epoch JD 2438895), noting that between JD 2415000 and 
2433300, 460.0 days was a better fit.  

This variation in reported period is 
unremarkable, given the cycle-to-cycle variations in period seen in many Mira 
variables.  However, since 1967, LX Cygni has apparently undergone
a significant change in pulsation behavior.  The American Association
of Variable Star Observers (AAVSO) archive of visual observations -- spanning 
JD 2439818 to 2452605 (November 23, 1967 to November 26, 2002) -- show that a 
dramatic increase in period has occurred since JD 2440000.  The period has 
grown from 460 days to over 580~days, an increase of nearly 25 percent.

For our time-series analysis, we used 961 visual observations by 84 different
observers in the AAVSO International Database.
Although we have additional CCD data taken in the Johnson {\it V} filter,
we chose not to include them in the long-term analysis in order to keep the
data set as homogeneous as possible.  We have instead analyzed the CCD
observations separately and discuss them below.
We used the {\it weighted wavelet transform} developed at AAVSO
(Foster 1996) to perform the time-series analysis.  The wavelet transform
allows one to measure the time evolution of the Fourier spectrum of a
given dataset, and it is quite sensitive to even small changes in period
over both short and long timescales.

We computed the wavelet
transform several times with a range of frequencies and wavelet windows.
We show the data and a representative wavelet transform in Figure 1, and plot 
the period and amplitude of the strongest peak as a function of time in Figure
2.  Figure 2 clearly shows that the period has increased since the start of 
the AAVSO data in 1967.  The behavior of the period is uncertain at the start 
and end of the data set due to edge effects of the wavelet transform.

We also note that the amplitude computed from the wavelet transform appears
to be variable over the span of observations.  It is possible that the real
amplitude of the star has changed over time, but this may be an artifact
of the wavelet analysis.  Wavelet amplitude is sensitive to data gaps, so
seasonal windows may be the cause.  Analysis of the amplitude variation is 
continuing, and will be published in a forthcoming paper.

\IBVSfig{7.5cm}{5367-f1.eps}{Wavelet transform of LX Cyg.  Contours 
represent the wavelet statistic, {\it w}, a measure of the statistical
significance of the signal (see Foster 1996).  Strongest signal lies at
the center of the contours.  The wider bands at the beginning and end of the
data stream are edge effects, and are artificial.}
%\IBVSfigKey{5367-f1.eps}{LX Cyg}{periodogram}

\IBVSfig{7.5cm}{5367-f2.eps}{Strongest period (solid line) and its 
amplitude (dotted line) derived from the wavelet transform of LX Cyg.  
The period is clearly increasing throughout the span of 
available data.  Because of edge effects in the wavelet transform, it is not
clear whether the period has reached a plateau 580 days, or whether it will
continue increasing.  Continued monitoring over the next several years is
strongly encouraged.  The amplitude also appears to be variable, and a
more sensitive analysis of this variation is underway.}
%\IBVSfigKey{5367-f2.eps}{LX Cyg}{periodogram}

To confirm our analysis, we also analyzed the available CCD 
{\it V}-band observations from the AAVSO International Database.  These 
observations consist of 106 CCD observations made by six observers, spanning 
JD 2451236 to 2452601 (February 26, 1999 to November 22, 2002).  
While the data span is not long enough to reliably determine whether the 
period is variable, we measured the spacing between the two maxima and three
minima present in the data and found a period of 590 $\pm$ 10 days.
In addition, we visually inspected the older data collected by 
Olivier et al.~(1940), spanning JD 2427334 to 2429595 (1933 to 1939).  
Although their data are very sparse, the measured separation between the 
two clearly defined maxima is 465 $\pm 10$ days, which is consistent with 
the published period for that epoch.  

The rapid period increase beginning at JD 2445000 appears to
be a continuous process, rather than an abrupt, discontinuous change.  
Therefore, it appears that mode-switching is {\it not} the reason for the
change.  If the period continues to increase at the current rate, than this
may indicate that LX Cygni is in the middle of a thermal pulse.  
According to the models of Vassiliadis \& Wood (1993), each thermal pulse 
begins with a sharp period {\it decrease}, followed by a short, high-amplitude 
oscillation in period (a decrease by half, and an increase by as much as a 
factor of three).  A period increase of 100 days over a few decades is seen 
in some of their models, and if LX Cygni is in the midst of a thermal pulse,
the period may continue to increase significantly in the coming decades. 

LX Cygni appears to be an excellent candidate for a Mira variable undergoing
thermal pulses, as the magnitude of the period change is similar (though 
opposite in sign) to that of T UMi, a star which has undergone a rapid
period {\it decrease} since 1968 (Mattei \& Foster 1995; G\'{a}l \& 
Szatm\'{a}ry 1995), as well as that of TY Cas (Hazen \& Mattei 2002).  Whereas
T UMi and TY Cas may have just begun thermal pulses, LX Cygni may be in a later
stage since thermal pulses are expected to begin with a period decrease.  
Further observations of LX Cygni are warranted and strongly encouraged.  A
more comprehensive analysis of this star is currently in preparation for
publication.\\
~\\

We thank the 90 observers worldwide who have contributed over 1000 
observations of LX Cygni to the AAVSO International Database.  We also thank 
G. Foster for helpful discussions on this analysis.


\references

Foster, G., 1996, {\it AJ}, {\bf 112}, 1709

G\'{a}l, J. \& Szatm\'{a}ry, K., 1995, {\it A\&A}, {\bf 297}, 461

Hazen, M. \& Mattei, J., 2002, {\it JAAVSO}, {\bf 31}, in press

Hoffmeister, C., 1930, {\it Sonn. Mitt.}, {\bf 17}

Kholopov, P. N., et al., 1985, {\it General Catalog of Variable Stars, 4th 
edition}, Moscow

Mattei, J. \& Foster, G., 1995, {\it JAAVSO}, {\bf 23}, 106

Olivier, C. P., et al., 1940, {\it Pub. of the Univ. of Pennsylvania}, 
{\bf 5}, part 3

Prager, R. \& Shapley, H., 1941, {\it Ann. Harvard College Obs.}, {\bf 111}

Semakin, N. K., 1955, {\it Peremenniye Zvezdy}, {\bf 10}, 100, (v.2)

Vassiliadis, E. \& Wood, P. R., 1993, {\it ApJ}, {\bf 413}, 641

\endreferences


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