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\IBVShead{4150}{26 January 1995}                      % Use it with dummy or empty arguments

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\title{IS TX DELPHINI A POPULATION I (CLASSICAL) CEPHEID?}

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\begintext


The pulsating variable TX Del (SAO\,16272, BD\,+3\degr 4437) is a
well-known, relatively bright ( $\langle$V$\rangle$ = 9.2 mag) Cepheid classified
as Type II according to the catalogue of Harris (1985). The
classification is based on its large distance from the galactic
plane: [z] = 450 pc if we assume d = 1.1 kpc distance from the
Sun derived from the period--luminosity relation of Type II
Cepheids (Harris, 1985). The assumption that TX Del is a
classical Cepheid would result in [z] = 1.2 kpc (Harris \& Welch,
1989). This latter [z] distance is much larger than the scale
height of classical Cepheids (70 pc, Fernie, 1968; Harris, 1985),
therefore Harris \& Welch (1989) concluded that TX Del is very
probably a Type II Cepheid.

There are some other criteria of the classification of Cepheid
variables, e.g. metallicity, galactic kinematics, light curve
shape, etc. (see Harris, 1985 and the references therein). The
classification, however, has large difficulties known for a long
time. Up to now the separation based on the [z] distances seems
to be the most useful and reliable. It is interesting that none
of these criteria support the Type II status of TX Del, because
it is a metal-rich star among Type II Cepheids ( [Fe/H] $>$ 0.5
according to Meakes et al., 1991) and its light curve is very
similar to those of classical Cepheids (e.g. Szabados, 1980).
Recently TX Del has been discovered to be a member in a
spectroscopic binary. Harris \& Welch (1989) separated the
pulsational and orbital radial velocities and determined the
orbital parameters of the system.

A project of computing physical parameters of metal-rich Type II
Cepheids via various Baade--Wesselink techniques has been started
by the authors. Unfortunately, the number of stars is quite low
due to the lack of precise radial velocity measurements but TX
Del is one of the best observed objects in this sample. There are
many light curves available and the precise pulsational velocity
curve determined by Harris \& Welch (1989) enabled us to compute
the mean radius with acceptable accuracy. The used data are
plotted against phase in Figure 1.

We applied two kinds of techniques deriving the radius: the
``classical" Baade--Wesselink method based on equal levels of the
colour index (B$-$V) (method \#1) and the surface-brightness method
based on the Barnes--Evans relation and the (V$-$R)$_0$ index (method
\#2). Both methods have lead to very similar results. Figure 2 shows
the result of velocity curve integration and the computed radii
at certain phases (using method \#1). The physical parameters
obtained are tabulated in Table I. E(B$-$V) = 0.1 was adopted from
Meakes et al. (1991).

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\centerline{2}
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\centerline{Figure 1 : Light-, color- and radial velocity curve of TX Del}
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\noindent Figure 2 : Radius variation of TX Del during a pulsational cycle.
The solid curve is the result of velocity curve integration, the
filled circles are the radii obtained from the
\centerline{Baade-Wesselink solution.}
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\centerline{3}
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\centerline{Table I}
\centerline{Baade--Wesselink parameters and inferred parameters of TX Del}
\vskip 1mm
\begin{center}
\begin{tabular}{ccccccc}
\hline
R/R$_{\odot}$&(B$-$V)$_0$&T$_{eff}$(K)&L/L$_{\odot}$ &M/M$_{\odot}$&d(pc)&z(pc)\\
\hline
\\
 48$\pm$2 &0.67&5500$\pm$200 &1940$\pm$300&6$\pm$0.3 &3000 &1200\\
\hline
\end{tabular}
\end{center}
\vskip 2mm
The obtained radius of TX Del (R/R$_{\odot}$ = 48) shows that this star
is too large to be a Type II Cepheid. The ``normal" radius of an
object of this type should be about 16 R$_{\odot}$ which is considerably
smaller. Moreover, the inferred radius is in very good agreement
with the period--radius relation for classical Cepheids (Gieren,
Barnes \& Moffett, 1989). Thus it is probable that TX Del is a
Population I (classical) Cepheid. Using this assumption we can
estimate the luminosity and the mass of TX Del as well as its
distance from the Sun and the galactic plane respectively. These
derived parameters are also summarized in Table I.


Since TX Del is a member of a binary system (the orbital period
is quite short, P$_{orb}$ = 133 days) it is important to compare its
radius with respect to the radius of the Roche-lobe. The mass
function of the system is 0.04 M$_{\odot}$ therefore the minimum mass of
the companion is 1.1 M$_{\odot}$. The Roche-lobe radius thus becomes about
110 R$_{\odot}$, so the ratio of radii R$_{star}$/R$_{Roche}$$\approx$0.44. This value
increases up to 0.5 if the companion is assumed to be as massive
as TX Del.


This work was supported by the Hungarian OTKA Grant \#F007318.
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% Call for macros \authors - \authorsrightcol - \endauthors
\beginauthors
\authorsrightcol
Z. BALOG and J. VINK\'O
Dept. of Optics
JATE University
Szeged, D\'om t\'er 9
H-6720 Hungary
\endauthors

% Call for macro \references
\references

Fernie, J.D., 1968, {\it AJ}, {\bf 73}, 995

Gieren, W.P., Barnes, T.G., Moffett, T.J., 1989, {\it ApJ},
{\bf 342}, 467

Harris, H.C., 1985, {\it AJ}, {\bf 86}, 719

Harris, H.C., Welch, D.L., 1989, {\it AJ}, {\bf 98}, 981

Meakes, M., Wallerstein, G., Opalko, J.F., 1991, {\it AJ}, {\bf 101}, 1795

Szabados, L., 1980, {\it Mitt. Sternw. Ung. Akad. Wiss. Budapest}, No.\,76
\IBVSlink{http://www.konkoly.hu/Mitteilungen/76.html}

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