Introduction

The double star CCDM 23239-5349 is very wide (angular separation of 26.5 arcsec). The Hipparcos satellite measurements confirm what was already made clear by the ground-based astrometric data catalogued at USNO, namely that such a wide separation is accompanied by a very small relative proper motion ( $\Delta \mu _{\alpha _{B-A}}\simeq -2.44$ mas/yr, $\Delta \mu _{\delta _{B-A}}\simeq +1.57$ mas/yr with errors of the same order) and a parallax that is compatible to better than $1.5\,\sigma$ ( $\pi _{A}=6.79\pm 1.43$ mas, $\pi _{B}=9.19\pm 2.44$ mas). This may indicate a common origin if not a real association. Regular short-period light variations on a time scale of $\simeq$ 5 hrs have been detected for the brightest component of this visual double star (Lampens, 1992). Apart from the interest of studying a pulsating star within a binary system, the detailed investigation of the difference in variability behaviour and physical parameters between the two components of a physical couple is particularly important when both stars have very similar colours: both are located in the same area of the colour-magnitude diagram, namely in the $\delta$ Scuti instability strip. Such a study may provide clues to better understand what factors determine the pulsation characteristics such as modes and amplitudes among $\delta$ Scuti stars in general. A more extensive version of this paper will be published elsewhere.