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\IBVShead{5039}{7 March 2001}

\IBVStitletl{Two New Short Period Variables:}{ HD 88278 and HD 128862}

\IBVSauth{Koen, C.$^1$; Abo-Elazm, M.S.$^2$}
\IBVSinst{South African Astronomical Observatory, P.O. Box 9,
Observatory 7935, South Africa}
\IBVSinst{National Research Institute for Astronomy and Geophysics,
Helwan Observatory, Cairo, Egypt}

\SIMBADobj{HD 88278}
\SIMBADobj{HD 128862}
\IBVSkey{photometry}
\IBVSabs{The results of short photometric runs on HD 88278 and HD 128862}
\IBVSabs{are presented. HD 88278 is probably monoperiodic, with a frequency}
\IBVSabs{of 15.72 c/d. HD 128862 is multiperiodic, the most prominent}
\IBVSabs{variation having a frequency of about 6 c/d. Both stars are probably}
\IBVSabs{pulsators of the Delta Scuti class.}

\begintext

Variability in both HD~88278 (HIP~49416) and HD~128862 (HIP72055) was
first discovered by the {\it Hipparcos\/} mission, with both stars
classified as ``Unsolved" variables (ESA 1997). Periodogram analysis
of the {\it Hipparcos\/} epoch photometry suggests single frequencies of
15.7~d$^{-1}$ (amplitude 21 mmag, HD~88278) and 22.7~d$^{-1}$
(amplitude 16 mmag, HD~128862) in the two sets of observations
(Fig.~1). This prompted us to obtain short observing runs on these
stars, in order to confirm the periodic variations.

All observations were made using the photoelectric Modular Photometer
attached to the 0.5-m telescope of the South African Astronomical
Observatory. Measurements consisted of a continuous stream of 10
second integrations through a $B$ filter, with occasional interruptions
in order to monitor the sky background. One short observation run (2.3
hours) on HD~88278, and two short runs on HD~128862 (durations of 2.7
and 3.5 hours), were obtained. Standard reduction procedures for such
observations were followed. The resulting light curves, consisting of
one minute averages of the reduced data, are shown in Figures~2 and~3.

A sinusoid with the frequency determined from the {\it Hipparcos\/} data
was fitted to the HD~88278 data by least squares, and is plotted with
the observations in Fig.~2. Given that no corrections to the data were
made for atmospheric transparency variations, the fit is quite good.
We conclude that the star is indeed variable with the {\it Hipparcos\/}
frequency of 15.72375 d$^{-1}$. (The least squares uncertainty is 4 in
the last quoted digit: the high accuracy is due to the long time
baseline of the {\it Hipparcos\/} observations). The amplitude in the
$B$-band is about 42 mmags.

\IBVSfig{11.5cm}{5039-f1.ps}{Amplitude spectra calculated for the {\it
Hipparcos\/} epoch photometry of HD~88278 (top panel) and HD~128862
(bottom panel). Prewhitening by subtracting appropriate sinusoids from
the datasets leaves residuals without any pronounced features in their
amplitude spectra}

\IBVSfig{7cm}{5039-f2.ps}{The $B$-band observations of HD~88278, and a
fitted sinusoid with a frequency of 15.7 d$^{-1}$}

\IBVSfig{10.5cm}{5039-f3.ps}{The $B$-band observations of HD~128862}

\vskip-36pt
It is clear from Fig.~3 that a single stable periodicity cannot
adequately describe the observations of HD~128862 --- for example, the
amplitude during the first run is considerably lower. A near-perfect
fit to the observations is possible with a four-frequency solution, but
given the paucity of data, much credence could not be put in it,
particularly as the amplitude of the star may simply be variable.
Least squares fitting of a single sinusoid gives a best frequency of
10.4 d$^{-1}$ (or nearby 1 d$^{-1}$ aliases). There is no exceptional
feature in the amplitude spectrum in Fig.~1 which corresponds to this
rather obvious frequency in our data. However, examination of the
window function of the {\it Hipparcos\/} data shows that strong aliases
may be expected at separations of $\sim 11.3$ d$^{-1}$ from the true
frequencies in the data, so that the most prominent feature in Fig.~1
(bottom panel) may be an alias of a true frequency of 11.4 d$^{-1}$.
The latter frequency is reconcilable with the results of our
observations.

The {\it Hipparcos\/} parallaxes of the stars, together with their $V$
magnitudes, give absolute visual magnitudes $M_V$ of $1.52\pm0.17$ and
$0.19\pm0.75$ for HD~88278 and HD~128862. These luminosities appear to
be respectively too low and too high for the MK classifications of
A4~III/IV (HD~88278) and F0~III/IV (HD~128862) (see Houk \& Cowley
1975). Agreement is better with the photometric absolute visual
magnitudes of 0.62 and 1.19 derived from Crawford's (1979) calibration
of the Str\"omgren photometric indices (Hauck \& Mermilliod 1998).
Other information derived from the Str\"omgren photometry is similar
$E(b-y)$ reddenings of 0.07 and 0.06, and $\delta c_1$ indices of 0.23
and 0.19, for HD~88278 and HD~128862 respectively. Finally, the
H$\beta$ and $c_0$ indices can be compared with the model results of
Moon \& Dworetsky (1985) to find $T_{\rm eff}=8000$~K, $\log g=3.6$
(HD~88278) and $T_{\rm eff}=7400$~K, $\log g=4.0$ (HD~128862).

The spectral types, periods and amplitudes of both stars are consonant
with their being $\delta$ Scuti pulsators. We therefore proceed to
calculate the implied pulsation constants $$\log Q = -6.456 -\log f +
0.5\log g + 0.1M_{\rm bol}+\log T_{\rm eff},$$ $f$ being the
frequency in d$^{-1}$. For HD~88278 we use the {\it Hipparcos\/}
frequency, while for HD~128862, $f \sim 11.4$~d$^{-1}$ is assumed.
Resulting pulsation constants are then 0.016, 0.013 (HD~88278) and
0.023, 0.030 (HD~128862), where the first value given for each star is
based on the {\it Hipparcos\/} absolute magnitude, and the second on the
photometrically derived $M_V$. These values are normal for $\delta$
Scuti stars.

\newpage

The authors thank Dr Gerald Handler for supplying an unpublished list
of candidate $\delta$ Scuti stars.
This research made use of the SIMBAD database, operated at
Strasbourg, France.

\references

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

ESA, 1997, The Hipparcos and Tycho catalogues, ESA SP-1200

Hauck, B., Mermilliod, M., 1998, {\it A\&AS}, {\bf 129}, 431

Houk, N., Cowley, A.P., 1975, Michigan Spectral Catalogue Volume 1,
University of Michigan, Ann Arbor

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

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