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\IBVShead{4828}{15 January 2000}

\IBVStitle{On Variability of R Sextantis}

\IBVSauth{MERCH\'AN-BEN\'ITEZ, P.; JURADO-VARGAS, M.}

\IBVSinsto{Departamento de F\'{\i}sica, Facultad de Ciencias, Universidad de Extremadura, Avda de Elvas s/n, 06071, \\ \indent Badajoz (Spain), e-mail: pedromer@unex.es, mjv@unex.es}

\SIMBADobjAlias{R Sex}{HD 84127}

\begintext

R~Sex (GSC 5474.380, HD 84127) was initially noted as a variable star at Harvard 105 years ago with a luminosity variation from 9.7 to 10.6 photographic magnitudes. R~Sex is included in the GCVS (Kholopov et al.\ 1985) as a doubtful semiregular variable star (Lb:) with a spectral type M5IIIe and without a period established. However, the variability of R~Sex was questioned and it was considered as ``constant?'' in the previous versions of GCVS. Due to the apparent stability in the brightness of R~Sex, Bidelman (1980) took into account spectroscopic observations to conclude that R~Sex does indeed deserve its variable star designation, due to the presence of hydrogen emission and its spectral variability. By using photometric observations, we demonstrate that the luminosity variation of R~Sex is real, and the star shows a light curve that could be consistent with its actual classification as Lb, although the classification as an SRd variable star could also be possible.

In order to confirm the variability of R~Sex, we made differential photometry in the $V$ band of the Kron--Cousins system during 18 nights, from 5 April to 10 June 1999, using a Starlight Xpress CCD camera (based on chip SONY ICX027BL $6.1 \times 4.35$~mm$^2$, $500 \times 256$~pixels) attached to the Newton focus of the 40-cm reflector at Observatorio del Departamento de F\'{\i}sica de la Universidad de Extremadura (Badajoz, Spain). The star GSC 5474.214 ($V=9.5$) was employed as a comparison and GSC 5474.444 ($V=9.7$) was chosen as a check star. The comparison star had constant brightness during the observations. Table~1 shows the CCD data obtained by us for each night for R~Sex and the comparison star. The mean accuracy of these observations was about 0.010 magnitudes. 

Figure 1 displays the light curve obtained for R~Sex. This light curve shows a rather flat minimum at HJD~2451308.4, and the most notable feature is an almost linear decline in brightness before the aforementioned date. The overall shape of this decline and the rise following it could be reminiscent of SRd behavior. Although the spectral type M5IIIe is not in a perfect agreement with the typical spectral types of SRd stars (roughly G to K), however, this type of variable stars usually shows strong hydrogen emissions, in accordance with the spectroscopic observations for R~Sex (Bidelman 1980). More information would be necessary to confirm this point. The essential characteristics of this type of stars can be found in the works of Dawson \& Petterson (1982) and Dupuy et al.\ (1983). Further observations of this star could confirm the possible new classification as an SRd star if their cycles show some regularity or, on the contrary, to conserve its actual classification as an Lb star if its light curve is irregular.

\begin{table}
\caption{Photometric data for R~Sex}
\begin{center}
\begin{tabular}{ccc}
\hline
\lower6pt\hbox{Mean HJD} & $\Delta V$ & $\Delta V$ \\[-3pt]
& (${\rm variable - comparison}$) & (${\rm comparison - check}$) \\
\hline
2451274.4 & 0.627${} \pm {}$0.014 & $-$0.218${} \pm {}$0.009 \\
2451276.4 & 0.640${} \pm {}$0.010 & $-$0.223${} \pm {}$0.008 \\
2451277.3 & 0.657${} \pm {}$0.010 & $-$0.221${} \pm {}$0.008 \\
2451279.3 & 0.656${} \pm {}$0.006 & $-$0.220${} \pm {}$0.007 \\
2451281.4 & 0.675${} \pm {}$0.007 & $-$0.223${} \pm {}$0.004 \\
2451284.4 & 0.687${} \pm {}$0.005 & $-$0.218${} \pm {}$0.004 \\
2451286.3 & 0.693${} \pm {}$0.009 & $-$0.222${} \pm {}$0.005 \\
2451292.3 & 0.724${} \pm {}$0.019 & $-$0.213${} \pm {}$0.016 \\
2451294.4 & 0.731${} \pm {}$0.007 & $-$0.212${} \pm {}$0.003 \\
2451308.4 & 0.764${} \pm {}$0.007 & $-$0.206${} \pm {}$0.008 \\
2451315.4 & 0.673${} \pm {}$0.002 & $-$0.221${} \pm {}$0.020 \\
2451317.4 & 0.649${} \pm {}$0.007 & $-$0.213${} \pm {}$0.013 \\
2451320.4 & 0.601${} \pm {}$0.009 & $-$0.220${} \pm {}$0.011 \\
2451327.4 & 0.477${} \pm {}$0.012 & $-$0.220${} \pm {}$0.013 \\
2451331.4 & 0.418${} \pm {}$0.015 & $-$0.206${} \pm {}$0.020 \\
2451333.4 & 0.394${} \pm {}$0.012 & $-$0.222${} \pm {}$0.009 \\
2451335.4 & 0.379${} \pm {}$0.009 & $-$0.214${} \pm {}$0.006 \\
2451340.4 & 0.278${} \pm {}$0.005 & $-$0.207${} \pm {}$0.020 \\ 
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10cm}{4828-f1.eps}{Photometric light curve for R~Sex}

{\it Acknowledgements:} This research was supported by the Consejer\'{\i}a de Educaci\'on y Juventud (Junta de Extremadura) and Fondo Social Europeo under project IPR98A047/

\references

Bidelman, W. P., 1980, {\it IBVS}, No.\,1858

Dawson, D.W. \& Patterson, C.R., 1982, {\it PASP}, {\bf 94}, 574

Dupuy, D.L., Allwright, W.B., Dawson, D.W. \& Africano, J.L., 1983, {\it PASP}, {\bf 95}, 427

Kholopov, P.N. et al., 1985, General Catalogue of Variable Stars, Nauka Publishing House, Moscow

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