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\IBVShead{5344}{26 November 2002}

\IBVStitle{Period Change in S Sextantis}

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

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

\SIMBADobj{S Sex}
\IBVStyp{ M }
\IBVSkey{photometry}
\IBVSabs{We present a wavelet time series analysis of the star S} 
\IBVSabs{Sextantis. The dominant period is shown to have varied}
\IBVSabs{over the past 90 years, though this variation has not}
\IBVSabs{been monotonic. We suggest that this star may belong} 
\IBVSabs{to the class of "meandering Miras."}


\begintext

S Sextantis is a well-observed Mira variable, with a mean period of roughly
260 days.  Merch\'{a}n Ben\'{\i}tez \& Jurado Vargas (2000) found a strong 
period decrease in S Sextantis based upon AFOEV and VSOLJ data spanning 
JD 2442000 to 2451340 (1973 - 1999).  They found that the period decreased 
monotonically from the 
GCVS period of 264.8 days to under 250 days, much larger than would be
expected from ``normal'' cycle-to-cycle variations often found in Miras.
From this, they hypothesized that S Sextantis had just undergone a helium
shell flash.  A few other variables are known to exhibit similar period
changes, with T UMi being the most spectacular case (G\'{a}l \& Szatm\'{a}ry 
1995; Mattei \& Foster 2000)

We analyzed the S Sextantis data from the AAVSO International Database, which 
is well-covered from JD2427871 to JD2452433 (1935 - 2002), to determine 
whether this period change began prior to JD2442000.  We used the 
{\it weighted wavelet transform} (Foster 1996) developed at AAVSO to study 
the time evolution of the pulsations.  The wavelet transform produces
a three-dimensional representation of the pulsation behavior with the
amplitude as a function of both time and period.

\IBVSfig{6cm}{5344-f1.eps}{Wavelet transform of S Sextantis.  Contours
represent amplitude in magnitudes.  The solid line at lower right represents
the span of the Merch\'{a}n Ben\'{\i}tez \& Jurado Vargas data.}

The results of the wavelet transform are shown in Figure 1.  We find that
for the span of time analyzed by Merch\'{a}n Ben\'{\i}tez \& Jurado Vargas, the
AAVSO data reproduces their observed period change.  However, we find that
the behavior of the period over time is far from monotonic, instead
appearing as a ``sinusoid'' with a period of roughly 50 years, and a full
``amplitude'' of the period change of nearly twenty days.  We also find 
that the amplitude of pulsation has changed nearly in lockstep with the
period, with lowest amplitudes corresponding to the shortest periods. 

The behavior of S Sextantis with time is strange in comparison to most Mira
variables.  Although Miras can exhibit very large {\it cycle-to-cycle}
period variations, they rarely exhibit changes that are as orderly as those
seen here.  Zijlstra \& Bedding (2002) make note of a small group of Miras
which exhibit long-term, orderly period changes, giving them the designation
of ``meandering Miras.''  In fact the pulsation behavior of S Sextantis is
remarkably similar to that of that of T Cep and S Ori, both of which show
evidence of the same sinusoidal variation in period.  However,
Zijlstra and Bedding note that at least 15~percent of Miras with periods
longer than 400 days show evidence of meandering periods.  Thus
it isn't clear whether S Sextantis' behavior has the same cause, or whether
this meandering behavior is limited to long-period
stars.  We show the period and amplitude variability of S Sextantis with T Cep,
R Nor, and S Ori in Figure 2.

\IBVSfig{8cm}{5344-f2.eps}{Period (solid lines) and amplitude (dotted 
lines) evolution in S Sextantis and three ``meandering Miras:'' T Cep, R Nor, and
S Ori.  Note how the period and amplitude changes appear to be correlated.}

Given the nature of the period variation in S Sextantis, it is unlikely that 
evolution is the cause.  The shell flash evolution models of Vassiliadis and 
Wood (1993) do not exhibit short-term oscillations in period like those
observed here; stars undergoing shell flashes have excursions in
period of several tens of percent, with timescales of $\sim 1000$ y. 
As yet, we have no explanation of what might cause this period variation.
Nonlinear interactions between two or more pulsation modes have been
suggested as reasons for chaotic behavior in the semi-regular and irregular
variables (Buchler et al. 1996), and a weakly-nonlinear process could be at 
work here.  Mode-switching does not seem possible since the period and 
amplitude variations are smooth, and the difference between maximum and 
minimum periods are smaller than the expected difference between radial 
overtones.  Interaction with a binary does not seem likely as there is no 
mention of S Sextantis being a close binary or symbiotic star in the literature,
although the sinusoidal nature of the variation is striking.  We suggest 
that S Sextantis is an interesting target for further study.


We thank the 288 observers worldwide who contributed the observations
that made this study possible.



\references

Buchler, J.R., Koll\'ath, Z., Serre, T., Mattei, J., 1996, {\it ApJ}, 
{\bf 462}, 489

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

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

Mattei, J.A. \& Foster, G., 2000, in {\it Variable Stars as Essential 
Astrophysical Tools}, C. Ibanoglu, ed., Kluwer: NATO Scientific Affairs
Division, {\bf 544}, 485

Merch\'{a}n Ben\'{\i}tez, P. \& Jurado Vargas, M., 2000, {\it A\&A}, {\bf 353},
264

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

Zijlstra, A.A. \& Bedding, T.R., 2002, private communication

\endreferences


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