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\IBVShead{4775}{13 October 1999}

\IBVStitle{A New {\Large$\gamma$}~D\lowercase{or} Candidate in Crux}

\IBVSauth{T. Arentoft and C. Sterken}

\IBVSinsto{Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussel, Belgium, \\ \indent e-mail: tarentof@vub.ac.be, csterken@vub.ac.be}


\begintext

In January and March 1999 the Dutch~91-cm telescope at ESO, La Silla,
was used for $uvby$ CCD observations of the Wolf--Rayet star WR~46. The
observations were carried out with a few data points each night during
28 nights distributed over three observing runs. During the data
reduction process we found that one of the other stars in the field
showed variability. This star is not listed as variable in SIMBAD or
in the GCVS (Kholopov et al.\ 1985--88), including the subsequent
name-lists (Kholopov et al.\ 1985, 1987, 1989; Kazarovets \& Samus 1990,
1995, 1997; Kazarovets et al.\ 1993).  The observed field is shown in
Figure~1, where the new variable star is marked by an arrow. Its
coordinates are $\alpha_{2000}=12\hr05\mm19\sec$ and $\delta_{2000} =
-62\deg03\arcm45\arcs$.  A light curve from March 1999 is shown in
Figure~2, together with the difference between the comparison stars.
The overall $rms$--scatter in the comparison star light curve is about
3.5~mmag.

\IBVSfig{8cm}{4775-f1.ps}{CCD frame covering the region where the new
variable has been found (arrow). The bright star 0\farcm5 north of the
new variable is WR~46.}

At least two of the nights had photometric conditions.  We used the
star HD~303308, in the nearby $\eta$~Car region, to determine the
zero--point shift to the standard system using the photometry of
Kaltcheva \& Georgiev (1993).  The average colour indices with
estimated errors are:
$$\begin{array}{r@{{}={}}l}
y & 14.54 \pm 0.02 \\
(b-y) & 0.43 \pm 0.02 \\
m_{1} & 0.04 \pm 0.03 \\
c_{1} & 0.71 \pm 0.05 \\
\end{array}$$

The errors on the observed indices are rather large, mainly due to
observational noise, but also due to the variability of this faint
object. The Str\"omgren colour indices are consistent with a late A or
early F star.

\IBVSfig{10cm}{4775-f2.ps}{Light curve of the new variable. Crosses
are the variable minus the average of two comparison stars, diamonds
the difference between the two comparison stars (with shifted
zero--point).}

Fourier analysis of the $y$ light curve reveals the presence of two
frequencies. The first is at 15.77~$\mu$Hz (1.363 c/d), corresponding
to a period of $\rm{17\hr36\mm30\sec}$, and with a semi--amplitude of
45.7~mmag. The second has a frequency of 17.93~$\mu$Hz (1.549 c/d,
$P=15\hr29\mm38\sec$) and a semi--amplitude of 23.5~mmag.  No
excess power is left in the amplitude spectrum after these two
frequencies have been subtracted from the light curve. The noise level
at low frequencies in the residual spectrum is about 4.2~mmag.  No
peaks higher than about 2~mmag are present in the amplitude spectrum of
the comparison stars.  \newline The spectral class, the
multiperiodicity, and the periods and amplitudes of the variations,
lead us to suggest that we are dealing with a variable star of the
$\gamma$~Doradus class.
 
$\gamma$~Doradus variables are a new class of pulsating stars
consisting of nonradial $g$-mode pulsators with periods between 8 hours
and about three days. In the H--R diagram they cluster around the
intersection of the red edge of the classical instability strip with
the main sequence (Handler 1999). Fig.~3 is the observational H--R
diagram for the 11 $\gamma$~Dor stars for which $uvby\beta$ photometry
is available, with $(b-y)_0$ and $M_{\rm V}$ determined by the method of Moon
\& Dworetsky (1985).  All known $\gamma$~Dor stars have
$2.705<\beta<2.767$ with an average $\beta=2.730 \pm0.022$. Using these
limits for the reddening--free $\beta$ index we obtain for the new
variable average $(b-y)_0=0.21\pm0.01, M_{\rm V}=2.8\pm0.6$ and $T_{\rm
eff}\sim 7100$~K, leading to the position in the H--R diagram shown by
the open symbol.

\IBVSfig{10cm}{4775-f3.eps}{
Observational H--R diagram: filled symbols are the
positions for known $\gamma$~Doradus stars, the open symbol 
denotes the location of the new $\gamma$~Dor candidate. The error bars 
reflect the uncertainty due to the expected $\beta$ range.
The full line is the ZAMS line given by Crawford (1979), the
dotted lines are the empirical borders of the $\gamma$~Dor locus as
given by Handler (1999).}

Our colour indices also yield a surprisingly high
$\delta_{m_0}\sim0.07$ (a measure of blanketing for a given $\beta$
that correlates well with metallicity [Fe/H]), the largest positive
$\delta_{m_0}$ value known for $\gamma$~Dor stars.

\bigskip

T.A. and C.S. acknowledge financial support from the Belgian Fund for
Scientific Research (FWO).  This research has made use of the SIMBAD
database operated at C.D.S., Strasbourg, France, and the NASA
Astrophysics Data System.

\newpage

\references

Crawford, D. L., 1979, {\it AJ}, {\bf 84}, 1858

Crawford, D. L., Mandwewala, N., 1976, {\it PASP}, {\bf 88}, 917

Handler, G., 1999, {\it MNRAS}, in press

Kaltcheva, N. T., Georgiev, L. N., 1993, {\it MNRAS}, {\bf 261}, 847

Kazarovets, E. V., Samus, N. N., 1990, {\it IBVS}, No.\,3530

Kazarovets, E. V., Samus, N. N., Goranskij, V. P., 1993, {\it IBVS},
No.\,3840

Kazarovets, E. V., Samus, N. N., 1995, {\it IBVS}, No.\,4140

Kazarovets, E. V., Samus, N. N., 1997, {\it IBVS}, No.\,4471

Kholopov, P. N. et al., 1985--88, General Cat.\ of Var.\ Stars, 4th
ed.\ (Nauka, Moscow)

Kholopov, P. N. et al., 1985, {\it IBVS}, No.\,2681

Kholopov, P. N. et al., 1987, {\it IBVS}, No.\,3058

Kholopov, P. N. et al., 1989, {\it IBVS}, No.\,3323

Moon, T. T.,  Dworetsky, M. M., 1985, {\it MNRAS}, {\bf 217}, 305



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