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\IBVShead{5529}{21 May 2004}

\IBVStitle{The Nature of the Eclipsing Binaries SS H\lowercase{ya} and VW C\lowercase{et}}

\IBVSauth{Lloyd, C.$^{1}$; West, D.$^{2}$}
\IBVSinst{Space Science \& Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, \\ Oxon.\ OX11 0QX, UK; e-mail: {\tt cl@astro1.bnsc.rl.ac.uk}}
\IBVSinst{West Skies Observatory, P.O. Box 517, Derby, KS 67037; USA, e-mail:{\tt dwest61506@aol.com}}

\SIMBADobjAliasJ2000{SS Hya}{HD 117408}{13 30 29.51 -23 38 58.2}
\SIMBADobjAliasJ2000{VW Cet}{2MASS 01390431-1750484}{01 39 04.31 -17 50 48.4}

\GCVSobj{SS Hya}
\GCVSobj{VW Cet}

\IBVStyp{ EB/KW: }
\IBVSkey{Eclipsing binaries - variable stars - photometry}

\IBVSabs{Both stars have been catalogued as beta Lyrae or W UMa binaries but}
\IBVSabs{practically nothing is known about them. An analysis of all the}
\IBVSabs{available photometry together with new observations suggest that}
\IBVSabs{neither star shows any periodic variation above 0.01-0.02 magnitudes}
\IBVSabs{but SS Hya may be a double-lined spectroscopic binary.}

\begintext
SS Hya and VW Cet are long-neglected eclipsing binaries that have had
suspicions raised about their variability. All the available photometry
has been reviewed and additional measurements have been made recently of
both stars by West using a 0.2-m f/6.3 Schmidt Cassegrain telescope with
ST8 and ST9E CCD cameras, and a V filter.

SS Hya (13\hr30\mm29\fsec51 $-$23\deg38\arcm58\farcs2) is bright, V = 7\fmm9
with a spectral type of A0\,V (Houk \& Smith-Moore, 1988) and is listed in
the GCVS (Kholopov et al., 1998) as an EA: system on the basis of over 900
photographic observations by Gaposchkin (1950), but no period is known.
Gaposchkin found a range of 8.10 to 8.30: pg, which is probably barely
larger than for constant stars, but made the comment, `eclipsing?'. The
star has a long history of suspected variability and was included in
variable star catalogues long ago. The discoverer, Bemporad (1911) found
variations between 7\fmm4 and 8\fmm1 from his visual observations of 1908--1911
and derived the elements $JD_{Max} = 2419207.4 +8\fday20\times E$, with
$M-m=3$d. Despite the brightness of the star there is very little modern
photometry. A small number of new measurements reported here give
$V=7\fmm89\pm0\fmm04$, which is generally consistent with the uncertainties.
Cousins \& Lagerwey (1970) made four measurements and derived mean values
of, $V=7\fmm88, (B-V)=0\fmm00, (U-B)=-0\fmm06$, with the comment, `Variable', and
although there is no indication of range, this comment does need to be
considered seriously.

Data are also available from Hipparcos and ASAS3. The Hipparcos data are
effectively constant at $Hp = 7.907 \pm 0.018$, with two apparently
discordant points 0.05 and 0.1~mag fainter than the rest. A Discrete
Fourier Transform periodogram (DFT) of all the Hipparcos data shows no
periodic variation greater than 0.015 mag for frequencies up to 
10~cycle\,day$^{-1}$, and without these two points the limit becomes 0.010~mag.

SS Hya is very close to the saturation limit for the ASAS3 instrument and
prior to a detector change at $JD = 2452200$ was in fact above it, which
sometimes produced spurious variations of up to 0.9 mag. The measurements
of SS Hya after this date, and with errors $<0.03$ mag, are effectively
constant at $V=7\fmm896\pm0\fmm011$. There are a few discordant points, up to
0.05 mag adrift, but the DFT periodogram shows no periodic variation
greater than 0.006 mag for frequencies up to 10 cycle\,day$^{-1}$.

\IBVSfig{8cm}{5529-f1.eps}{The light curve of SS Hya from the Hipparcos 
data (left) and the ASAS3 data (right). All the ASAS3 data prior to $JD = 
2452200$ are suspect due to saturation problems and many of the fainter
points lie off the plot.}
\IBVSfigKey{5529-f1.eps}{SS Hya}{light curve}

In summary the photometry suggests that SS Hya is not an obvious
variable; at most the full amplitude must be $< 0.03$, which is well
below the range of photographic or visual detection. The only remaining
doubt about its lack of variation is the comment by Cousins \& Lagerwey,
otherwise there would be no suggestion that it was variable at all.

Adams et al. (1924) give two radial velocities, apparently for two
components, of $-98$ and +117 km\,s$^{-1}$ with the comment, `Double
lines. Doubtless {\it Algol} type'. If it is an Algol system then the
secondary is sufficiently bright to be seen spectroscopically. In
contrast, the General Catalogue of Stellar Radial Velocities (Wilson,
1953) gives a mean velocity of +10 km\,s$^{-1}$ with the poorest quality
flag (E), based on just two measurements. If the large velocity variation
and double lines are correct then the system must contain stars with
similar luminosities and not too dissimilar masses. If the components are
both approximately A0\,V then a period, $0.5 < P < 2$ days is required to
generate the velocity range, depending on the inclination. As there are
no obvious eclipses the longer periods are physically impossible, and
with $P \sim 1$ day an inclination, $i < 40$ is required to avoid
eclipses, which is not unrealistic. If it is an Algol system then
everything depends on the mass ratio, but both stars will still have
broadly similar sizes and the inclination requirement will remain
approximately the same. It therefore seems possible that SS Hya is a
low-inclination binary, possibly showing very low-amplitude ellipsoidal
or grazing eclipse variations.

The Hipparcos parallax of SS Hya is $5.80\pm1.07$ mas which leads to
1-$\sigma$ absolute magnitude of, $1\fmm25 < M_V < 2\fmm06$. Interstellar
extinction should be small as at $<200$\,pc the star is only just beyond
the local bubble (Sfeir et al., 1999). A ZAMS A0\,V star is expected to
have $M_V=1.5$, and more general values are up to a magnitude brighter,
which leaves little leeway for an additional component. A significantly
less luminous secondary might be accommodated, but unless the parallax is
badly in error, two stars of similar luminosity are not possible.

\IBVSfig{8cm}{5529-f2.eps}{The light curve of VW Cet showing the
unfiltered ROTSE data (open circles) and the ASAS3 V data (filled
circles).}
\IBVSfigKey{5529-f2.eps}{VW Cet}{light curve}

\newpage
VW Cet (01\hr39\mm04\fsec31 $-$17\deg50\arcm48\farcs4) is listed in the GCVS
(Kholopov et al., 1998) as a short-period eclipsing binary of uncertain
type, EB/KW:. It was discovered by Petit (1953) who reported a
short-period eclipsing variable of $\beta$ Lyrae or W UMa type with a
visual range of 11\fmm6 to 12\fmm1. These variations were apparently confirmed
by 35 observations by Oskanjan (1953), with a visual range of 11.6 to
12.2, and a period about 0.24 days. Finally, Petit (1956) derived the
published ephemeris $JD_{Min} = 2435111.396 + 0.486\times E$ based upon
110 visual estimates. There have been no further measurements until very
recently.

Our new observations of VW Cet have been obtained serendipitously since 2001
during a programme to monitor UV Cet, which lies close by. These 96
observations have \mbox{$V=11\fmm979\pm0\fmm064$}, which is broadly consistent with the
uncertainties, but they show no suggestion of a 0.486 day variation. A DFT
periodogram of these data is essentially noise with no periodic variation
greater than 0.025 mag for frequencies up to 10~cycle\,day$^{-1}$.

VW Cet has also been observed by ASAS3 and ROTSE. Limiting the ASAS3 data
to those with errors $<0.03$ mag and ignoring one discordant point
(leaving 197 points), gives $V=11\fmm930\pm0\fmm039$. A DFT periodogram of
these data shows no periodic variation greater than 0.021 mag for
frequencies up to 10 cycle\,day$^{-1}$. Similarly, restricting the
ROTSE-1 data to those with errors $<0.03$ mag (115 points) gives\\
\hbox{$M_{ROTSE}=11\fmm943\pm0\fmm020$}, and the maximum periodic variation from the
DFT periodogram is 0.011 mag. A very similar limit is also found for the
combined ASAS3 and ROTSE data sets.

In summary there is no evidence for the 0.486 day period for VW Cet and
there is no reason to believe there is any periodic variation at all. The
total extinction in this direction from the NASA/IPAC Extragalactic
Database is very low, $A_V=0\fmm06$ so the 2MASS colours $J-H = 0\fmm46$(3) and
$H-K = 0\fmm08$(3) should be very close to intrinsic, and suggest that VW Cet
has a spectral type close to K0. It has a small but significant proper
motion so it is most likely a field dwarf.

West's photometry of SS Hya and VW Cet is available through the IBVS
website as {\tt 5529-t1.txt} and {\tt 5529-t2.txt} respectively.

\IBVSedata{5529-t1.txt}
\IBVSdataKey{5529-t1.txt}{SS Hya}{photometry}

\IBVSedata{5529-t2.txt}
\IBVSdataKey{5529-t2.txt}{VW Cet}{photometry}

\noindent
Acknowledgements

The authors in indebted to Nikolai Samus for providing the historical
background on these stars and to Grzegorz Pojmanski for a discussion of
the saturation problem with ASAS3 data.

\references

Adams W.S., Joy A.H., Sanford R.F., 1924, {\it PASP}, {\bf36}, 137 

Bemporad A., 1911, {\it Astron. Nachr.}, {\bf189}, 225, (No. 4524)

Cousins A.W.J., Lagerwey H.C., 1970, {\it MNASSA}, {\bf29}, 7 

Gaposchkin S., 1950, {\it Harvard Ann.}, {\bf115}, No. 15

Houk N., Smith-Moore M., 1988, {\it Michigan Spectral Survey}, {\bf Vol. 4}

Kholopov P.N., Samus N.N., Frolov M.S., Goranskij V.P., Gorynya N.A.,
Karitskaya E.A., Kazarovets E.V., Kireeva N.N., Kukarkina N.P., Kurochkin
N.E., Medvedeva G.I., Pastukhova E.N., Perova N.B., Rastorguev A.S.,
Shugarov S.Yu., 1998, {\it The Combined General Catalogue of Variable Stars},
4.1 Edition.

Oskanjan V., 1953, {\it Belgrade Obs. Bull. V.}, {\bf17}, No. 3-4, 35-36

Petit M., 1953, {\it Documentation des Observateurs. Bulletin}, No. 4

Petit M., 1956, {\it Letter to GCVS}

Sfeir D.M., Lallement R., Crifo F., Welsh, B.Y., 1999, {\it A\&A}, {\bf346}, 785

Wilson R.E., 1953, {\it General Catalogue of Stellar Radial Velocities}, Carnegie
Inst. Washington D.C. Publ., {\bf 601}

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